Ok, my prediction is based on this:
In Goroawase 3 can be read as follows:
Mittsu, Mi, San, Sa, Za, Su, Suri.
Whereas 9 can be read as follows:
Kokonotsu, Ko, Kyu, Ku, Gu, Nain.
So 3.9 billion would be San Kyu/Ku
It would have to be 9,3 for Kyu/Ku San. Whether it's 9.3 billion, 930 million all the way down to 93 berries.
So "93".
Now, I can't say I am as sure as I was for Akainu's 5B because for that there were multiple hints (the fact that the system mirrored Doflamingo's, that there were surely 5 crowns as a max for various reasons, etc.), but:
- Kuzan's bounty should start at the very least from 3B, but likely higher;
- Kuzan has captured Garp, a 3B marine but more so a living legend, the fucking Hero Garp in the flesh. In OP terms, Is something incredibly big that even BB hyped up as incredible. Not to mention the symbolism behind It;
- why don't reveal It now? We have already seen 3,5-6 for a subordinate, so if It was less or even 3,6-7, why Oda hasn't?
Imho, It's because It wasn't that amount, but an amount very near to BB's if not superior to him;
- Aokiji is one of the most important characters of OP and now that Akainu Is at 5, having Aokiji the step lower seems more likely. So Akainu at five and Aokiji at ~ 4;
- last but not least, at this point I think Aokiji's bounty will be released only when BB reaches 5B. Aokiji could have done something more by that point, be It against the RHP or with the AW that BB Is going to seize.
For all these reasons, I think his bounty could very well be near to 4 if not beyond 4. I am not 100% as I was with Akainu that he will be above 4, but I am 100% sure that he is going to have 3.893B ("93" Is Kuzan Is Goroawase) at the very least, if not directly 3.93B.
I would be willing to bet on anything starting from 3.893B, which anyway would be barely 100 million shy of the 4B. Which for a subordinate would be unheard of considering that we had full fledged Emperors captains with less or barely more. And that Is the botton line, imho.
For the sake of the poll, I'll say 4B, even though 3.983 and 3.93 I think are possible too as I said.