This discussion has been coming up on fb so I thought it needed a post here!
So fans are divided regarding Yonkou v Admiral power levels.
Some believe that they are equal - and it is probably because Admirals are strongest on one side, and Yonkou are strongest pirates, so in the same verse, they should be equal power. They reference WB being lucky not to die sooner in Marineford, as reason why admirals are stronger...
Some think Admirals are stronger. This is not true. Kaido is confirmed strongest in the world lol BM is a match for him (from their Greeting at Onigashima). Either of them would solo an Admiral.
It is pretty clear that Yonko are stronger than Admirals and it doesn't appear that way for one reason: Age. We know from Don Chinjao, Lao G, Garp etc. that Haki fades with age (Luffy beat Chinjao, who rivalled Prime Garp so I think their fighting power is reduced bu 80%) with advanced age. We know WB was old, sick, and wounded when fighting those Admirals, but its not just him ...
None of the Yonko are Prime Power. BM Kaido andShanks are past Prime (Yonkou are weakened) and Luffy is not yet Prime. BB maybe*, but he doesn't fight. He relies on the Yami Yami no mi and running
So the discrepancy here come from the stage of the era that they are in. The Yonkou are experiencing a changing of the guard, while the Marines have trained and implemented new Admirals (the big 3) as well as drafting new ones.
Even so, they are pretty evenly matched. In general though, the Yonkou would be stronger than the Marines!