The four pirates who hold the New World in balance, and the clearest demonstration in manga of a number used as a power ceiling.
Filed by its count, not its title
Its count and its English name happen to reduce alike, so both routes reach 4 · The Structure. The count is still what files it — the agreement is a coincidence, not the reason.
Four pirates whose individual strength is treated as roughly equivalent and jointly as the upper limit of the world's pirate power. They do not form an organisation, meet, or acknowledge each other's authority. They are a category imposed from outside by the World Government, which is precisely what makes the number work.
Four is a plateau, not a ladder. Unlike a ranked set, the Emperors carry no internal order — no reader asks who is the first Emperor, because the number is describing a tier rather than a queue. That gives the story a stable ceiling it can measure everything else against, and it means removing one member is a structural event rather than a promotion.
It also solves a pacing problem. A protagonist climbing toward a single final enemy has one destination and one shape; four equivalent apex figures give the same climb four possible routes, and the author can defer choosing until the story needs one.
Four is the number Japanese convention usually avoids: 四 is read shi, a homophone of 死, death, and buildings routinely skip the fourth floor. Villain quartets are nonetheless everywhere in manga, and the older Four Heavenly Kings is the reason. Whatever superstition says about lifts, a four of enemies reads as complete and faintly funereal, which is exactly the register a top tier of pirates wants.