Its nothing special. I just plagiarized from the manga One Piece.
The are two seas; the New World and the Grand line. The Grand line, basically for amateur pirates. The New World, on the other hand, is where the grand treasure "One Piece" lies. The pirates are scattered all around these two seas. Their counterparts are the marines.
The four emperors are the most powerful of all pirates and each control a large portion of the New World. Consequently, they are the closest to the grand treasure.