I assume 3.9. Only because I'm assuming Blackbeards does not increase again and Shanks' stays exactly where it is. I also think Akainu will have 4 crowns, to match the rest of them.
Then Luffy gets into a huge war with everyone (including CG) and between that war and whoever survives and makes it to Lodestar/LT, he ends up with a higher bounty than Roger.
Basically 2 more increases