To be honest, I wasn't expecting Mihawk bounty to be above 3B
12 years ago - Mihawk and Shanks had duels which shook the grandline.
It's clear that they had multiple duels because it was kind of draw otherwise there was no need to have so many duels.
Then shanks lost his arm - and According to manga Mihawk lost interest in dueling shanks.
So, to me their bounties by this time should be almost equal atleast.
11 years ago Mihawk became a warlord so his bounty got frozen
6 years ago Shanks became yonkou - I think shanks bounty definitely got jumped due to this and reaching to his present status of 4B. I am expecting atleast One Billion Jump in his bounty
Now, if Mihawk bounty is around 3.5 or above now then it could means mihawk could have slightly higher bounty than shanks 11 years ago
And, if Mihawk bounty is around 3B then it fits well with both shanks and Mihawk having almost equal bounty 11 years ago because of their duels