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 Emporers 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.

The shift in the Balance
So far two inccidents committed by the Straw Hat Pirates have seemed to disrupt this balance in favour of the Emperors:
 * 1) When Crocodile of the Shichibukai was defeated, Shanks managed to make contact with Whitebeard.
 * 2) The inccidents at Enies Lobby shifted the balance against the Marines. Whether it directly effected events playing out in 'the New World' or not, soon after this inccident Whitebeard and Shanks broke through an attempt to hault both Emperors progress to see each other.

List of Pirate Emperors

 * 1) Shanks
 * 2) Whitebeard
 * 3) Unknown(possibly kaidou because he was strong enough to defeat gecko moria and is also in new world)
 * 4) Unknown