We're talking about a former admiral, who was initially supposed to be promoted to Fleet Admiral. Who the world knows clashed with Akainu for 10 days straight, and barely lost, both coming out with permanent scars. Who then went rogue, but not only that, went and allied himself with the Yonko Blackbeard.
If that's not enough for 4 bil, I will laugh. This dude should be seen as a traitor, especially with the marines under Akainu's lead, and the fact that he was an admiral and so knows the inner workings of marines, and their tactics and dispositions and whatnot, makes him a very valuable tool for any pirate, much less a Yonko. And then you add on his well known strength.
On top of recent events, such as his involvement in capturing Garp The Hero.
The literal only reason he would not have 4 bil, like 3.93 as people are saying, Is because Oda does not want Shanks to be topped by any admiral, to make him look better until his inevitable downfall vs Blackbeard