If Kaidou was under someone he might not even have 4 bill.
Yes, but Rayleigh's combat feats should speak for themselves. For instance, if the fact that he was called the "Dark King" is purely based on combat achievements... I don't think Rayleigh having his wings clipped while being under Roger's command would affect him that much. It would be sufficient for Oda to depict him fighting against opponents who are not as weak as those Roger faced.
If Roger faced Garp, and Rayleigh faced Sengoku, thus maintaining consistency, but considering the way Oden was approached by Rayleigh and Gaban, it makes sense for Rayleigh's bounty to be lower than Whitebeard's, Roger's, and perhaps even Kaido's, the monster.
Regarding Kaido, well, the issue is that he doesn't serve anyone, so it's not possible to compare that to Rayleigh. And Kaido's bounty apparently is higher than Rayleigh's, given that it's mentioned that no one has ever exceeded the bounties of Roger and Whitebeard, which implies that Rayleigh's bounty is below Whitebeard's 5 billion, even after finding the One Piece, Rayleigh's bounty did not surpass that of Whitebeard.
You think 1.5 Billion is the boost for finding the OP?? One hell of a number
Yes, how much were they offering for Robin just for being able to read the Poneglyphs? Knowing the entire history, all the atrocities the government committed in the past, plus the credit for having reached the treasure, the One Piece, it should be in the range of 1 to 1.5 billion.