Yonkou aka the Four Emperors are four most powerful pirates in the world. They are the third group of One Piece’s balance of power, the other two being the World Government and the Shichibukai. World Government article is here, and the Shichibukai’s is here.
Unlike the Shichibukai, the Yonkou are not affiliated with the World Government nor the Marines, and they are also not part of the Revolutionary Army. They are the truest form of pirates. Prior to the 2 year time skip, the members of the Yonkou includes:
Edward “Whitebeard” Newgate, formerly known as the most powerful man in the world. During the age of Gold Roger, he was considered the 2nd most powerful. He has numerous pirates following him and owns number of territories, including the Fishmen Island. Luffy’s brother, Portgas D. Ace, was a commander of his.
Red Haired Shanks, not much is known about what he did after he left Fusha Town. He can go toe to toe with Whitebeard and was once a member of Pirate King’s crew. Not much is known about Shanks, but he was a former member of Pirate King’s crew and probably knows where the grand treasure One Piece is.
Kaidou the Hundred Beast is the most recently revealed Yonkou. According to the narrations, he is impossible to kill, even by the World Government. He is trying to create the most powerful all-fruit-user pirate crew. And since Luffy took down Doflamingo during Drerossa arc, that plan has been crushed, so he is keen on destroying the Straw Hat pirates.
Big Mom is now the most mysterious Yonkou, as we have only seen her face covered in heavy shadow. But from what we’ve seen, she looks gigantic, and loves sweets. Luffy has declared war on her over the territorial right to the Merman Island, and as of now, she has Sanji in her possession.
After the time skip, Marshall D. Teach aka Blackbeard replaced Whitebeard as one of the Yonkou and is no longer part of the Shichibukai. Somehow, he was able to steal Whitebeard’s earthquake ability and stands as one of the most powerful pirates.
Yonkou are probably the most powerful individual group in the world. To demonstrate, the combat Whitebeard and his pirate family (several pirate crews working under his banner), the World Government had to gather the Marines (their most powerful branch), all of the Shichibukai, and experimental Pacifistas. This was to take down one old and injured Yonkou who is probably long past his prime.
In one of the most recent chapters Kaidou mentioned that the new generation thinks their hot shit just because they topple a Shichibukai or two. And he has Eustass “Captain” Kidd beat up in a sell. Kidd had the highest bounty of the Worst Generation prior to the time skip, even higher than Luffy.
Hope you liked it,