I wonder who the fourth yonko was before Shanks? It must have been someone with a bounty of ≈ 3.8-4B considering the small gaps between BM, Kaido and WB. When Shanks took him out he himself got 4B + change. Because if there was no fourth yonko then it means Shanks must have stolen islands from WB, Kaido and BM. There can not have been independent islands. As soon as WB died BM stole FI.
Ochoku or whatever his name had Beehive, I guess he could have been the one who used to be a yonko before Shanks forced him to retreat leaving with only Beehive until BB took that as well. Apart from him and Shiki there are no candidates at all.