Just to be clear for the OP: it sounds like you are looking at having PVG-LAX and LAX-LAS on separate tickets? If so, there is some extra risk. If you bought PVG-LAX-LAS all on one ticket, then the airline is responsible to rebook you if anything goes wrong (delays on the inbound flight and you miss your connection, massive line at customs, etc.).

4 hours is enough time to give you some buffer, but on separate tickets there is a chance that if your PVG-LAX flight is significantly late and you miss your connection, you could be in trouble. Delta may take pity on you and help you rebook anyway (especially because you're also flying in on a Delta flight), but they don't have to.

You'll probably be fine, but just worth being aware that there is some extra risk if you're on separate tickets.