You need three full days in Siem Reap, so:
--day/night 1, arrive and get oriented
--day/night 2, temples
--day/night 3, temples
--day/night 4, temples
--day 5, leave
Luang Prabang: there are about 5 things to do in LPB and environs, but time moves pretty slowly in Lao and 4 nights might be just fine.
I'd carefully weigh the probability of seeing rare wildlife and the certainty that some of the less visited ruins in Thailand, Laos, and Cambodia will only be more visited as tourism is developed.