Four Emperors.
It could refer to the Four Emperors themselves or to their forces.
The Four Emperors ruled over the New World, the latter half of the Grand Line, like emperors. Their fame, influence, and strength were all extremely close to, and comparable with, the overlord from before the Great Pirate Era—the Pirate King, Gol D. Roger.
The Four Emperors' pirate crews could be called the four strongest pirate crews of the Great Pirate Era. They were also enormous mountains that pirates had no choice but to face during their normal voyages along the Grand Line.
Pirate crews that had made it through the challenges of Paradise, the first half of the Grand Line, had only a few choices upon entering the New World when they encountered the Four Emperors: challenge them, join them, be destroyed, or turn back.
As a result, the Four Emperors had also gathered many pirates and subordinate crews under their banners. That was why they were called pirate emperors.
There were only two paths to becoming the Pirate King.
Either reach the final island and find the One Piece left behind by Pirate King Roger, or completely defeat the Four Emperors and more than half the pirate crews in the world.
The former still offered a slim chance of luck. The latter required absolute military might, using strength to crush all opposition and forcefully ascend to the top.
Even someone as strong as Roger had only been called the Pirate King after reaching the final island, Laugh Tale, and sailing around the world once.
In terms of strength, Roger had been unable to defeat everyone either.
When someone one day emerged on the seas and defeated the Four Emperors and every strong person in the world with absolute force, the title of Pirate King would