Yonko = individual power first the rest comes second.
BB said its early for Luffy to be the fifth Yonko, he obviously meant individual strength.
BB become Yonko when he beat Marco. Before he wasn't despite having the same crew and army.
Elizabello's King Punch was hyped by Yonko's individual strength.
Its a power level proven by Oda. Yonko=Top level individual strength, then crew, army etc comes second.
Yonko Pirate Emperor title is a much better title than WSS title.
You can keep your WSS title if a non-swordsman pirate kicks your ass. You are still WSS if you lose to someone like Jozu, Marco or Katakuri.
You can't keep your Yonko title if any pirate beats you, such as if Marco beat BB or if Katakuri beat Luffy.
Its even arguable that being Luffy's right hand > WSS because Zoro or Sanji can't lose to Marco or Katakuri at the EoS while WSS can lose and still remains as WSS.