Tbh I think in typical shonen fashion they will be put beyond the "typical" Yonko threshold. Think about it.
The 2 current ones being tackled and Whitebeard are significantly older. Once they are done, BB and Shanks can't realistically be put at the same level as them. They have to be scaled stronger for the purpose of making more final enemies/battles (Blackbeard at least and I personally believe Shanks is an antagonist but we'll see).
In that regard we should probably just end the idea of a Yonko post-Wano. It probably won't fit the mold anymore