I overstayed in the UK then voluntary left the country four years ago. Now I got a USA tourist visa and am planning to go on a vacation in California and Las Vegas. Will my previous overstay history affect my ability to enter the USA? Will the US immigration find out about my overstay history? My USA visa is in a new passport.
If you left the UK voluntarily four years ago (and the authorities did not know you overstayed, such as by asking you to leave), then most likely they still don't know about it. The UK did not introduce formal exit controls until recently.
If you have a USA visa already approved, then the US immigration either doesn't know or doesn't care about your overstay in the UK.
In the landing interview (where you talk to an officer after arriving in the US), they can ask you any question including whether you have overstayed in any other country. This would be an unusual question for them to ask at that point, but if they do, always answer truthfully.