Luffy coats his arm in Busoshoku Haki before biting into his forearm. Similar to how he activates Gear Third, he blows an incredible amount of air into his body, but this time he inflates his muscular structure before distributing the air throughout his body, with emphasis on his upper half. If Luffy is stopped mid-blowing, the arm will slowly deflate.
When utilizing Gear Fourth, Luffy makes extensive usage of his Busoshoku Haki by coating his arms, legs and much of his torso in it to the point that they turn black from hardening. The Haki coating across his body resemble the wispy ends made from flames, giving it the appearance of tattoos, and he gains shading around the inner and outer edges of his eyes. He also physically grows taller, standing around 4 meters tall. Similar to when using Gear Second, Luffy is continuously emitting steam in this form. Combined with Luffy's rubbery composition and his compressed muscle structure, Gear Fourth both adds explosive power to Luffy's attacks and grants him enhanced defensive capabilities far beyond what he can demonstrate with or without Gear Second or Gear Third. Depending on the situation, Luffy can adapt Gear Fourth into different forms.
Using Gear Fourth has a number of drawbacks for Luffy. There is a limit to how long he can keep Gear Fourth active, as noted by both Luffy himself and Doflamingo. Once this limit is reached, Gear Fourth automatically deactivates, leaving Luffy exhausted and barely able to move. He is also left unable to use Haki for ten minutes after using this technique as it quickly depletes his Haki reserves. The technique also greatly taxes Luffy's stamina, as using it just once leaves Luffy so physically drained in the aftermath that he can barely stand and fight, even after he has regained his Haki and mobility. Even afterwards, Gear Fourth leaves its marks on Luffy's body. Akin to Gear Second, using Gear Fourth causes Luffy's metabolism to increase rapidly. In fact, after Luffy stuffed himself to a bloated size in his fight with Charlotte Cracker, using Gear Fourth once made him burn off the massive amount he had eaten in a manner of minutes. Furthermore, using Gear Fourth briefly for a second time in a row once rendered him unconscious for three days. Using Gear Fourth also makes him more prone to falling asleep. As such this "Gear" is completely a last resort if Gear Second and Third fail to affect his opponents. However, the drawbacks has shown signs of diminishing. While retreating from his battle with Katakuri when his first attempt to use Gear Fourth against him failed, Luffy could still run and escape the Mirro-World even after his Gear Fourth wore out. He was still fast enough to dodge a surprise attack from Amande and he could still use his attacks, though he could not imbue them with Haki. He also had enough stamina to flee from a food-crazed Big Mom.
While Luffy is using Gear Fourth, his physical strength and speed are increased to the point where he is able to overwhelm a member of the Shichibukai, Donquixote Doflamingo, and a high ranking member of a Yonko crew, Charlotte Katakuri, whereas before, Gear Second's lack of strength and Gear Third's lack of speed made them ineffective in the fight.
Unlike the previous forms, Gear Fourth focuses on compression more than stretching. He can compress his arms, legs and head. While the compression grants Luffy's moves overwhelming speed and power, the range of his attacks is severely decreased. For example, the Gomu Gomu no Bazooka is a technique that Luffy has proven he can execute over varying distances, but due to needing to actually compress his fists for the Leo Bazooka, it required him to be right in front of Doflamingo. When the latter unveils his Devil Fruit Awakening, Luffy expresses his annoyance at his difficulty getting close to the Shichibukai. However, he can still deliver long-ranged attacks by stretching the part of the body he wants to attack with while retaining great power and speed, as his body is still greater in size and covered in Busoshoku Haki. Luffy also shows a much more versatile side of the Gomu Gomu no Mi's powers by changing the direction of his attacks mid-strike, by bouncing off the air, to hit a moving opponent without losing momentum.
According to Doflamingo, the power of Luffy's attacks is greatly magnified in this form. His durability is also enhanced: despite being hardened with Busoshoku Haki, his body is still rubbery, so physical blows (even ones enhanced with Busoshoku Haki) simply bounce off of him and\or used as a counter attack, depending on the form Luffy use.
In the anime, the luster of the Busoshoku black parts and the Busoshoku tattoos take on a reddish tint. Furthermore, Luffy seems to take up some kabuki-like mannerisms, like exaggerated head-rolling, and occasionally dragging out words. Luffy's voice also gets rough and little deeper.
Luffy's first Gear Fourth form is called Boundman (弾む男 (バウンドマン) Baundoman?, literal meaning "Bounce Man"; Viz and Funi subs: Bounce Man): Luffy's proportions become warped and his body becomes much larger in size, with his arms, upper torso, and legs coated in Busoshoku Haki. As a side-effect, he is incapable of standing still on the ground, and instead constantly bounces on the spot. This form was created in order to combat the many powerful and large animals on Rusukaina.
This form, much like Gear Second, seems to utilize the principles of Rokushiki. It possesses both high speed movement reminiscent of Soru and the ability to kick off the air, similar to Geppo, cleverly adapted to work even better with his body's elasticity: Luffy not only bounces off the air, but allows attacks to bounce off his own body. His body is also now strong enough to defend against cutting attacks head-on.
After Boundman lands on the ground after using an attack, Luffy cannot seem to control his landing due to warped proportions his body takes and bounces like a ball in a comical fashion until he either loses momentum or he somehow regains control and starts "flying" again.
Though he has some heightened resistance to cutting attacks in this form, if an opponent using a bladed weapon has Busoshoku Haki of comparable or greater strength to Luffy's own, they will be able to cut through his coated arms with relative ease. Similarly, his near-immunity to blunt force attacks is also not absolute. If an opponent has powerful enough Haki in conjunction with their physical strength, it's possible to negate the rubbery defense of Luffy's inflated body and injure him.
Due to the increase in Luffy's metabolism Gear Fourth causes, he is able to leave his Tankman form in a short time after using it. This is the case when Luffy was able to burn off the massive amount of biscuits he had eaten in his fight with Cracker shortly after he defeated the Sweet Commander.
Currently, the base form of "Tankman" is unknown, since the only time it has been used was after Luffy ate Cracker's Biscuits to the point where he was completely full and used his "Manpuku Version" form.
During his battle with Charlotte Katakuri, Luffy revealed another form of Gear Fourth known as Snakeman (蛇男 (スネイクマン) Suneikuman?, literally meaning "Snake Man"). Unlike his other two Gear Fourth forms, Luffy does not significantly expand, with his forearms and legs being only slightly enlarged and the rest of his body retaining its normal proportions. The spread of Haki is slightly more limited, as it does not appear to extend onto Luffy's shoulders and upper body. However, Luffy's hair becomes flame-shaped like his Haki and sticks up. The steam produced from his body also seems to be draped over his shoulders like a scarf while in this form. His fists also flex differently, with the first knuckles being extended, similar to leopard fist.
In this form, Luffy gains a significant increase in speed and ability to use "Python" to change the direction of his attacks, the latter of which he also possesses in his Boundman form. However, Snakeman allows his attacks to continuously accelerate the longer they last, allowing Luffy to not only press an attack more fiercely on an opponent capable of dodging, but increase the attack's speed and power until he finally lands the hit, and it also gives him the opportunity to attack any area of an opponent with ease. This form focuses on dealing with agile opponents who specialize in dodging and requires Luffy's recently heightened proficiency in Kenbunshoku Haki in order to make his punches into semi-homing attacks.
This form also allows Luffy to jump decent heights, as shown when he leaped out of a chasm in Mirro-World, though its jumping ability compared to Boundman is unknown.
However, because of his body's lack of inflation, Luffy's defense is considerably worse compared to his Boundman and Tankman forms. When he was thrown into structures by Katakuri, he took the impact like he normally would, as opposed to bouncing like he does in Boundman. Additionally, he receives blunt attacks like he does normally rather than having them bounce off him.
- Kin'niku Fusen (筋肉風船 Kin'niku Fūsen?, literally meaning "Muscle Balloon"): By coating his arm in Busoshoku Haki and biting into his forearm, Luffy is able to blow a large quantity of air into his body and inflate his muscular structure, with the air blown into his arm spreading out to the rest of his upper body. In this form, his durability is enhanced to the point where a Busoshoku Haki enhanced kick from Donquixote Doflamingo, a Shichibukai, bounced back harmlessly, due to his body retaining its rubber properties despite being coated in Haki.
- Python (大蛇 (パイソン) Paison?, literally meaning "Serpent"): Using a combination of Busoshoku Haki and his rubber body, Luffy is now able to redirect his punches during an attack instead of retracting it back, allowing himself to bypass to necessary build up time between attacks. In his Snakeman form, Luffy claim the attack speeds up as the attack goes on.
- Gomu Gomu no Kong Gun (ゴムゴムの猿王銃 (コングガン) Gomu Gomu no Kongu Gan?, literally meaning "Rubber Rubber Monkey King Gun"): The Gear Fourth version of Gomu Gomu no Pistol. After compressing his fist into his enlarged forearm (and making it look like the barrel of an actual cannon), Luffy unleashes a devastating short-range punch with massive power. This technique had enough strength to break through Doflamingo's own Haki-enhanced defense and hurl the Shichibukai all the way from Dressrosa's palace to the middle of the city. This technique is drastically more powerful than anything Luffy has utilized in Gear Second or Gear Third, best demonstrated when he was completely unable to harm the 860,000,000 bounty Cracker with Hawk Gatling or Elephant Gun, having them blocked and deflected by his shields, but sent him reeling with a Kong Gun while also easily shattering the shields. Later, this technique also had enough strength to break through Katakuri's own Haki-enhanced defenses and hurl the Sweet Commander away when they fought in the Mirro-World. However, it was easily blocked by Charlotte Linlin when she hardened her elbow as well. "Kong" refers to King Kong, the titular gigantic gorilla from the film of the same name. In the Viz Manga and FUNimation subs, this is called Gum-Gum Kong Gun.
- Gomu Gomu no Rhino Schneider (ゴムゴムの犀榴弾砲 (リノシュナイダー) Gomu Gomu no Rino Shunaidā?, literally meaning "Rubber Rubber Rhinoceros Howitzer"): The Gear Fourth version of Gomu Gomu no Yari. After compressing both legs into his body, Luffy delivers a double flying kick. It has enough power to send the opponent crashing through dozens of buildings and across a town center. This was first used as a surprise attack against Doflamingo. A howitzer is a type of short-barreled artillery cannon, and "Schneider" refers to the Canon de 155 C modèle 1917 Schneider, a howitzer used by several countries during World War I. In the Viz Manga and FUNimation dub, this is called Gum Gum Rhino Schneider.
- Gomu Gomu no Culverin (ゴムゴムの大蛇砲 (カルヴァリン) Gomu Gomu no Karuvarin?, literally meaning "Rubber Rubber Great Serpent Cannon"): The Gear Fourth version of Gomu Hook. A culverin is a relatively simple firearm, ancestor of the musket and cannon. In the Viz Manga and FUNimation subs, this is called Gum-Gum Culverin.
- Gomu Gomu no Double Culverin (ゴムゴムのダブル大蛇砲 (カルヴァリン) Gomu Gomu no Daburu Karuvarin?, literally meaning "Rubber Rubber Double Great Serpent Cannon"): The Gear Fourth version of Gomu Gomu no Twin Pistol. Luffy pulls back his arms before punching his opponent with them. This was first used against Doflamingo, but was countered by his Offwhite.
- Gomu Gomu no Leo Bazooka (ゴムゴムの獅子 (レオ)・バズーカ Gomu Gomu no Reo Bazūka?, literally meaning "Rubber Rubber Lion Bazooka"): The Gear Fourth version of Gomu Gomu no Bazooka. After retracting both of his fists into his arms, Luffy "fires" them at his opponent with a double open-palm strike. This was first used while airborne against Doflamingo, and its power sent him flying over Dressrosa and caused him to create an enormous crater when he slammed into the side of the King's Plateau. In the Viz Manga and FUNimation subs, this is called Gum-Gum Leo Bazooka.
- Gomu Gomu no King Kong Gun (ゴムゴムの大猿王銃 (キングコングガン) Gomu Gomu no Kingu Kongu Gan?, literally meaning "Rubber Rubber Great Monkey King Gun"): The Gear Fourth version of Gomu Gomu no Gigant Pistol and Gomu Gomu no Elephant Gun, and one of the most powerful techniques Luffy has ever utilized in any form. After Luffy compresses his fist, like he does for Kong Gun, he blows a large amount of air into his arm, drastically increasing its size, then unleashes it for a devastating blow. This was first used to defeat Doflamingo, and was powerful enough to break through the Shichibukai's Spider's Web and God Thread techniques simultaneously, while still maintaining enough momentum to send him flying into the city below with enough force to completely level the town and cause the ground to fold in on itself. Using this technique causes the Haki patterns on Luffy's back to expand and cover his entire back with thicker patterns. In the Viz Manga and FUNimation subs, this is called Gum-Gum King Kong Gun.
- Gomu Gomu no Kong Organ (ゴムゴムの猿王群鴉砲 (コングオルガン) Gomu Gomu no Kongu Orugan?, literally meaning "Rubber Rubber Monkey King Murder of Crows Cannon"): The Gear Fourth version of Gomu Gomu no Gatling and rapid-fire version of Kong Gun. After compressing both fists into his arms, Luffy moves his arms horizontally in a rapid fashion, with the afterimages giving the impression of two extra pairs of arms being materialized, before releasing a flurry of continuous heavy punches. This was first used against Cracker's Biscuit Soldiers, and each individual punch was powerful enough to smash through a biscuit shield. In the Viz Manga, this is called Gum-Gum Kong Organ Gun.
- Gomu Gomu no Leo Rex Bazooka (ゴムゴムの獅子王 (レオレックス)・バズーカ Gomu Gomu no Reo Rekkusu Bazūka?, literally meaning "Rubber Rubber Lion King Bazooka"): The Gear Fourth version of Gomu Gomu no Gigant Bazooka and Gomu Gomu no Grizzly Magnum. Before Luffy compresses his fists like he does for Leo Bazooka, he blows a large amount of air into his arms, drastically increasing their size, then unleashes it for a devastating blow, similar to King Kong Gun. This was first used to defeat Gild Tesoro. In the FUNimation dub, this is called Gum-Gum Leo Rex Bazooka.
- Gomu Gomu no Jet Culverin (ゴムゴムのJET大蛇砲 (ジェットカルヴァリン) Gomu Gomu no Jetto Karuvarin?, literally meaning "Rubber Rubber Jet Great Serpent Cannon"): A faster version of Boundman's Gomu Gomu no Culverin. Luffy compresses his fist and releases it, using Python to change the direction of his punch according to the actions of his opponent. His arm speeds up the more it stretches, and the force of the punch was strong enough to smash Charlotte Katakuri into a wall. It was first used against Katakuri.
- Gomu Gomu no Black Mamba (ゴムゴムの黒い蛇群 (ブラックマンバ) Gomu Gomu no Burakku Manba?, literally meaning "Rubber Rubber Nest of Black Snakes"): Luffy arches backwards and compresses both arms before unleashing a rapid-fire strike, moving his arms up and down to give the impression of multiple arms moving to attack. As with Jet Culverin, Black Mamba involves Luffy changing the direction of his attacks, but their rapid movements make Black Mamba harder to avoid. Unlike its counterpart in the Boundman form, the Kong Organ, Luffy only manifests two pairs of afterimages instead of three pairs, indicating a lower frequency of attacks. However, he makes up for it with the semi-homing abilities of the Python, though it is not as accurate as the Culverins. It was first used against Katakuri, though he managed to dodge or block the majority of the attacks.
- Gomu Gomu no King Cobra (ゴムゴムの王蛇 (キング・コブラ) Gomu Gomu no Kingu Kobura?, literally meaning "Rubber Rubber King Snake"): Luffy compresses his arm, and as he releases it, instead of directly targeting his opponent, he first has his arm loop around himself once before sending his enlarged fist flying at his opponent. The wide loop gives his attack additional acceleration, thus increasing its momentum, while enlarging the fist adds more mass, the combination multiplying the force of this strike. It was first used against Katakuri, successfully knocking him out in a single hit.
Tankman: Manpuku Version
The first time this form was shown, Luffy had consumed copious amounts of biscuits and consequently grew to an enormous and bloated size, and he referred to this version of Tankman as Manpuku Version (満腹バージョン Manpuku Bājon?, lit. Full Stomach Version). In this form, Luffy resembles a sumo wrestler with exaggerated proportions: while his arms and legs are slightly increased in size with muscle, his Busoshoku Haki-coated torso becomes far larger and completely round, though still muscular near the top. The flame-shaped Haki also covers the bottom of his torso in addition to his shoulders and arms. This form also bears resemblance to Luffy's appearance when he combined the strength of both his Gear Second and Gear Third into one while fighting Gekko Moriah (Gigant Jet Shell). In the Viz manga it is called "Tankman: Stuffed Version".
Tankman gives Luffy tremendous Haki-based durability: while Cracker nearly cut through Luffy's Haki-coated Boundman arm with his sword, he was unable to pierce Luffy's stomach with the same sword while the latter was in Tankman form. Furthermore, this form further increases the power and magnitude of Boundman's compression-based offense by using Luffy's enormous stomach as a launcher, which allowed him to defeat Cracker by blasting him through his incredibly hard Biscuit Soldiers with Gomu Gomu no Cannonball.
The known Devil Fruit attacks that Luffy uses in this form are as follows:
- Gomu Gomu no Cannonball (ゴムゴムのキャノンボール Gomu Gomu no Kyanonbōru?): The Gear Fourth version of Gomu Gomu no Fusen and Gomu Gomu no Orei Fire. After trapping an enemy within him by compressing and withdrawing his stomach, Luffy launches them forward and into the air with incredible power and speed. This technique was first used to defeat Charlotte Cracker, who, when launched, plowed through several of his immensely hard Biscuit Soldiers and was sent flying far away.
- In Volume 79's SBS, it is revealed that Gear Fourth is based on the fierce guardian entity called the Nio Statue, which fits well with Luffy's will to protect his crewmates with his life and beyond.
- The anime added some details during the Boundman transformation which were absent in the manga:
- In the anime this is the first of Luffy's forms to have a soundtrack made up especially for it. The theme is called "Gear Fourth" and is part of the One Piece New World Original Soundtrack.
- Luffy´s Devil Fruit, the Gomu Gomu no Mi, can be seen in the background while his torso is being coated with Busoshoku Haki.
- The steam generated seems to be semi-gaseous, resembling the ribbon of the Nio statue (inspiration of Gear Fourth). This behaviour of the steam is included later in the manga.
- Similarly to Gear Second, Gear Fourth allows Luffy to use techniques that appear to be his own re-imagining of Rokushiki abilities, his pseudo-flight bears a striking resemblance to Geppo, and Leo Bazooka is seemingly reminiscent of Rokuogan (with Gear Second allowing Luffy to move at a speed similar to Soru).
- The concept of retracting limbs inward was first utilized by Kaku's Kirin Hodai.
- Luffy being covered by Haki in almost all his body, except for the abdomen, chest and head and the Haki shadow trim around his eyes are similar to Dragon Ball series, Super Saiyan 4 (instead of Haki, the Super Saiyan 4 is covered by hair). In addition, both forms are named after the number 4.
- Boundman resembles his appearance when he became Nightmare Luffy (except for the hardened Busoshoku Haki 'tattoos', the enlarged biceps and a more rounded torso) due to Luffy's increased muscle size, haircut's shape and sinister expression.
- The position of Luffy's hands to perform Leo Bazooka highly resembles the Kamehameha stance from the Dragon Ball series, a manga that highly inspired Oda.
- Excluding Culverin and its variations, all Boundman's techniques are named after mammals, whereas Snakeman's techniques after snakes.
- Ironicly, the word "culverin" is derived from couleuvre, the French word for grass snake.
- Even though it is only one technique, it seems that Tankman’s techniques are named after heavy weapons.
- Rubber - Wikipedia article about rubber
- Yubari King - Wikipedia article about the actual fruit the Devil Fruit is apparently shaped after
- Rubber Man - TV tropes wiki article on the rubber super power trope
- Firearm - Wikipedia article about firearms in general for which a majority of Luffy's Devil Fruit techniques are named after