1. Luffy
2. Law
3. Kidd
4. Crocodile
(assuming we are going with an actual Yonko system as well as Pirate King is a Yonko).
Excluding Luffy...
1. Law
2. Kidd
3. Crocodile
4. Marco
No real order in strength, but just listing them.
"4 Emperor's" won't exist when there's a PK because they exist since they equally govern the NW in quarters and are equally the strongest pirates and the pirates closest to obtaining OP. And the PK is someone who has conquered all - which includes all the islands on their journey to reaching Raftel. Hence once there's a PK, there shouldn't be other people with the title of "Emperor".Luffy
Law
Marco
Sir Crocodile
Without Luffy
Law
Marco
Sir Crocodile
Akoiji OR kid