Okay, let me leave this question here:
There's the Shichibukai, Yonko and Marine HQ are the Three Great Powers in the world of One Piece, but people like Luffy and Law and Kid and Zoro take them on (mostly Luffy) most of the time to the point where it almost cause world crisis or whatever when Moriah and Crocodile lost to Luffy and got revoked. So technically the Worst Generation should also be part of that and have it be the Four Great Powers. And eventually is the Yonko and Shchibukai are gone, they'll takeover and stuff. So what is there no Four Great Powers?