He can be weaker than the 4, doesn't matter.
Point is Oda and Kaidou think he's comparable.
Kaidou and Oden even back then were Top Tiers.
Same for Shanks & Mihawk when they had their "Legendary" duels.
You gotta be a Top Tier first to become a Yonko, your Powerlevel doesn't suddenly spike up when you are granted a title.