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.
Luffy's proportions become warped and his body becomes much larger in size, with his arms, upper torso, and legs coated in Busoshoku Haki. The coating across his torso 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. 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. It 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. This form also 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.
On Volume 79's cover, Luffy's skin is shown to take a pinkish tone (similar to Gear Second in the anime). However, in the anime, though Luffy's uncoated skin is unchanged, the Busoshoku tattoos take on a reddish tint. Furthermore, he seems to take up some kabuki-like mannerisms, like exaggerated head-rolling, and occasionally dragging out words.
However, due to his characteristics upon entering this form, his enemies may see it as a joke and start to mock him, as seen when Doflamingo laughed at this form upon seeing it.
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, 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. 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.
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.
However, there appears to be a limit to how long Luffy can keep Gear Fourth active, as noted by both Luffy himself and Doflamingo. Once this limit is reached, Gear Fourth is automatically deactivated, 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. Even afterwards, Gear Fourth is extremely taxing on Luffy's body and stamina; after regaining his Haki from using Gear Fourth once, he barely had enough strength left to fight back. After his brief use of it the second time, it rendered him unconscious for 3 days. After this, he seemed to mimick Ace's habits, namely his narcolepsy. As such this "Gear" is completely a last resort if Gear Second and Third fail to affect his opponents.
Also, after he lands on the ground after using an attack, he 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.
- 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.
- 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"): 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 Third, best demonstrated when he was completely unable to harm the 860,000,000 bounty Charlotte 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 shattering one of the shields. The kanji's literal reading is "En'oju". "Kong" refers to King Kong, the titular gigantic gorilla from the film of the same name.
- 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"): After compressing both legs into his body, Luffy delivers a double flying kick, similar to Gomu Gomu no Yari. 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. The kanji's literal reading is "Sai Ryudanpo/ho"; the latter kanji term being the Japanese name for a howitzer. 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.
- 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"): Similar to Afro Luffy's Gomu Hook, but improved. Luffy delivers a punch like a regular Gomu Gomu no Pistol, but if it is dodged, he can redirect his punch an unspecified number of times, without losing momentum, to still land with tremendous force. In addition, this punch does not need to retract back to his body right after being "fired" as the regular Gomu Gomu no Pistol does. In the anime, the stretching arm is referred as Python (大蛇（パイソン） Paison?, literally meaning "Serpent"). A culverin is a relatively simple firearm, ancestor of the musket and cannon. The kanji's literal reading is "Orochi/Daijahou"; Orochi is the name of the eight-headed serpent of Shinto mythology.
- 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"): 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. This was first used against Doflamingo. The literal kanji reading for "Leo" is "Shishi".
- 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"): 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. The kanji's literal reading is "Dai En'o Ju".
- Gomu Gomu no Kong Organ (ゴムゴムの猿王群鴉砲 (オルガン) Gomu Gomu no Kongu Orugan?, literally meaning "Rubber Rubber Monkey King Murder of Crows Cannon"): The rapid-fire version of Kong Gun. After compressing both fists into his arms, Luffy moves his arms horizontally in a rapid fashion before releasing a flurry of heavy punches in a manner reminiscent of Gomu Gomu no Gatling. This was first used against Cracker's Biscuit Soldiers.
- In this form, Luffy's Busoshoku Haki takes the shape of tribal flames on his body. This is the only example of Busoshoku Haki forming a recognizable pattern.
- The anime added some details during the transformation absent in the manga:
- While changing to this form, Luffy is seen with a red aura around him.
- 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).
- 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.
- Also, 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.
- 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).
- 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 Original Soundtrack -New World-.
- 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