HistoryMuch of the history of the Yonkou is unknown.
The first of the Yonkou to be introduced was Shanks who visited Luffy's hometown ten years ago. Although, it is unknown that the Yonkou existed at this point of time. It is reasonable to assume they did as this was the around the time the Shichibukai organization was being formed. The second was Whitebeard, the only man to tie in a fight with Roger. Whitebeard was also later announced as the strongest pirate in the world upon Roger's death. Some of the Shichibukai were rivals to the Yonkou in the past; Shanks was a rival to Mihawk, Kaidou to Gecko Moria, and Whitebeard to Crocodile.
Shanks and Whitebeard are both familiar to the old days of piracy, before Roger's death 22 years ago. It is currently unknown if the other two Yonkou are from the old days of piracy.
All of the Yonkou seem to have strong fleets, including ships and elite crewmen. All Yonkou have traveled to the New World at some point in their past, where they are currently locked in stalemate, unable to progress any further into the Grand Line. This is a product of the combined efforts of both the Marines and Shichibukai, as well as the presence of the other Yonkou in the Grand Line.
Until recently, the World Government has been fine with the world powers remaining as they are. With the Yonkou stuck in a deadlock situation, this allowed the world to remain stable. However, with the absence of Crocodile, defeated at the hands of the Straw Hat Pirates, the World Government was alarmed that two of the Yonkou, Shanks and Whitebeard, arranged to meet each other. Later, with Enies Lobby in ruins, the Marines saw another defeat in the New World when a blockade failed to prevent the two meeting.
Fearing the two crews might form an alliance, the World Government lay in wait to see what happened. Shanks believes the days of Whitebeard as the top pirate are coming to an end, warning him of the era to come. With Blackbeard aka Marshall D. Teach now making his move to the top, the pair are last seen clashing weapons.
In an attempt to rescue his 2nd Division Commander, Portgas D. Ace, Whitebeard is currently attacking Marineford. Kaidou attempted to stop Whitebeard. However, he was intercepted by Shanks.
StrengthThey are one of the 3 great powers, the other being the Shichibukai and Marines. They live quite literally as though they are Emperors, doing mostly as they please. For the World Government, they are the greatest pirate threats in the world. It is only the combined efforts of the Marines and the Shichibukai that keeps their power in check. Interestingly, their existence as a disunified group appear to be even less pronounced than that of the Shichibukai, having only been introduced right after the introduction of Garp
The four emperors themselves are not united together as one group like the Shichibukai and follow their own individual paths. However, the possibility that two or more of the Yonkou could form an alliance is a great threat to the World Government. For that reason, the World Government attempts unsuccessfully to prevent a meeting between Red-Haired Shanks and Whitebeard at one point in the series. The World Government feared that the two emperors may unite together and create a stronger threat. A war between two of the 4 Yonkou is considered a epic crisis. Shanks' interception of Kaidou's advancement on Whitebeard had taken the Marines by shock and awe.
They can inspire others to become pirates. Examples of this include Shanks inspring Luffy and Whitebeard inspiring Ace. On top of this, the influence these pirates have over others is far greater then any other noted group. With one declaration, the kidnappings on Fishman Island stopped after the island was announced to be under Whitebeard's control.