How?
Koby is 500 mil. By this scaling the Admirals will all be in the 2 or 3 billions at worst.
Its stated that any normal captain has 1 star i.e 100mil.
Coby's bounty being 500mil is because of his reputation as "Hero" even tho he is captain.
If im not mistaken captain and a VA difference is around 4-5 positions. So any regular VA should be around 500-600mil. Scaling through that the Admirals bounty would be around 3 times this. So as a base they would have 1.5 bil.
To have above this would be due to reputation which would be max around 3bil.
The only possible admiral with bounty close to 4bil would be Sakazuki because he is FA.
For this CG would need funds close enough to WG to hand out bounties in billions which i dont think they have. Remember there are way too many ranked marines. If even captains are getting bounties then their funds are getting spread thin.