Mihawk has a legitimate excuse for having a bounty lower than Shanks. Because NOT ONLY he has no crew and all kinds of territories. His bounty was FROZEN for a LOOONG TIME. Bounty does not jump in one time! Bounty increases gradually. The WG may give him a high bounty, but for him to reach the 4 bill + he needs his own crew, territories and alliances, as well as he needs to have MORE THAN ONE EVENT. Meaning, Luffy increased in bounty from East Blue, then Alabasta, then Enies Lobby, then Marineford, then Dressrosa, then WCI. Bounty does not jump to te sky let alone 4 billion from just abolishing Shichibukai System. So his excuse is VALID 100%
Hell, Look at BB who has EVERYTHING Mihawk lacks from crew, territories, and he still is far below because he's relatively NEW YONKO.
But Shanks has NO EXCUSE for having lower bounty than Kaido/BigMom. The excuse that Shanks holds back and doesn't hurt towns...etc CANNOT BE USED because WB was maintaining the peace of the sea and STILL had higher bounty than all current Yonko.
It only simply means that as of right now, the 4 current Yonko's bounty is ranked in accordance to strength and NOTHING ELSE (maybe you can exclude BB for being new, but even despite that, I expect him to be weaker regardless of bounty because he's new Yonko)