Not sure I understand your post lol
We know that some bounties are a result of pirates being Chaotic/destructive. Shakky talks about this when she explains why Kid's bounty was higher than Luffy's.
Again, Kaido and BM being at 4.6 and 4.3 has nothing to do with "power". It's all factored into threat with the WG. They are violent pirates with a pent up desire to destroy shit, and have been doing so for 40-50 years.
BB and Shanks are relatively young in comparison. BBs is low because, like Luffy, he started from no bounty only 2 years prior. It's going to go up, since he's probably just as bad or worse than Kaido/BM.
Shanks is the odd one. He's at 4 Billiion and has only been a Yonko for 6 years. Yet, out of all Yonko, has the propensity to NOT cause conflicts. He's all about peace.
The question is why does he have this threat level to the WG if he's stopping wars for Sengoku and chatting with Gorosei? It's very odd.