I think different states have different laws, and in some states the answer to that question is not clearly spelled out. Utah seems to be one of them. But, I asked a cop that same question when I lived in Utah and according to him, Car B has the right of way. He told me that technically, Car A could get stuck there forever if traffic cleared briefly allowing one car to go, then cross traffic got heavy again - Car C could show up and go straight across the next time traffic cleared, making Car A wait again.

However, I know that many people think it should be Car A. I bet the majority might even say Car A. That doesn't make them right (they're probably wrong), but it does mean that if you're Car B, you should probably be careful. Even if you have the right of way, you don't want to wreck your car. People get confused by the 4 way stop rule and think there's something special about stopping first. If Car A is in the intersection and turning left, Car B has to yield. But since both are stopped and waiting, Car A yields when it clears up.