Four Emperors



The Yonkou (四皇, Yonkō), literally Four Emperors, are the 4 great pirates who rule the second half of the Grand Line (known as 'the New World').

They are one of the 3 great powers, the other being the Shichibukai and Marines and live quite literally as though they are Emperors, doing mostly as they please. For the World Government, these are the greatest pirate threats in the world. It is only through the combined efforts of the World Government and the Shichibukai that keeps their power in check. Each of them is equal in strength to Whitebeard and whenever there is a shift in the balance of the 3 powers one of the 4 Pirate Emperors makes a move.

The 4 Emperors themselves are not united together as one group like the Shichibukai and live by their own individual paths in life. For that reason, the World Government does not want a meeting between Red-Haired Shanks and Whitebeard, fearing the 2 Emperors may unite together creating a stronger threat.

History
About strength, the yonkou themselfes can be compared in strength with the Shichibukai members, since two Yonkous, Shanks and Kaidou, had been rivals of two Shichibukais, Mihawk and Gecko Moria respectively. Although, so far the Yonkou have shown a crew incredible stronger compared to a Shichibukai crew, for example Portgas D. Ace, one of Whitebeard division commanders, when defeated granteed the title of Shichibukai for Blackbeard.

So far three inccidents committed by the Straw Hat Pirates have seemed to disrupt the 3 great powers balance in favour of the Emperors, while one caused by Blackbeard repaired this balance to a certain degree:
 * 1) When Crocodile of the Shichibukai was defeated, Shanks managed to make contact with Whitebeard.
 * 2) The incidents at Enies Lobby shifted the balance against the Marines. Whether it directly affected events playing out in 'the New World' is unknown. Soon, after this incident, Whitebeard and Shanks broke through attempts to halt both Emperors from seeing each other.
 * 3) With the defeat of Gecko Moria (another Shichibukai) the balance is disrupted even more, though Gecko Moria's defeat is largely unknown to the public.

List of Pirate Emperors

 * 1) Shanks
 * 2) Whitebeard
 * 3) Kaidou
 * 4) Unknown