How exactly bounties are roughly estimated in the OP world have always been all over the place.
Generally Aokiji told us that Bounty reflects:
- Combat abilities
- Thread to the WG
Now the 2 part can again be divided into multiple sub-categories on its own which makes them so confusing.
Affiliations, Political power, possession of certain weapons etc.
However, we as long time readers can definitely say that Oda is using bounties as portrayal, hype and in many examples power levels.
- Katakuri was coincidentally the confirmed strongest sweet commander with the highest bounty.
- King will have the highest bounty of the calamities and going from the card theme we know that he also will be the strongest etc etc...many examples.
Shanks having a 4B bounty with 39 is indeed impressive, but we can't deny that in comparison to the other "original" Yonkou his took kind off a hit, especially since Shanks fans took pride in the opposite due to false editor hype.
Comparing bounties in a specific age is also not much of use, tbh. 39 Year old is prime Shanks on the peak of his pirate life, just like the other Yonkou. I doubt that his bounty will increase at from this point onwards.