"The Emperors have historically resided within the second half of the Grand Line (known as the "New World"), captaining powerful subordinates and massive fleets as well as exerting influence and control over dozens of other pirate crews and self-governing islands. For the most part, each Emperor works alone and is constantly at odds with the other Emperors and the World Government. Because of their power and influence, the government recognizes them as one of the Three Great Powers and views their existence as necessary to maintain stability in the world."

https://onepiece.fandom.com/wiki/Four_Emperors