1. Find someone who has moved into a new house.
(In this sentence, does the person live in the new house or no?)
2. Find someone who has had more than two dogs. (In this sentence, does the person have two dogs now? Or does he have ever had two dogs before, and he doesn't have them now.)
3. Find someone who has been on a diet.
(In this sentence, does the person is on diet now or did the person finish the diet?)
4. Find someone who has been in the hospital.
(According to the sentence, is the person in the hospital now?)
1. If the person has moved into the new house, he/she is undoubtedly living in it.
2. With the verb "has had," it is asking about someone who had two dogs in the past and the person may still have two dogs in the present, or maybe not. It's not clear cut because it's asking about the past not the present.
3. This is about the same as #2 -- someone who has been on a diet in the past and could still be on a diet now -- or maybe not.
4. Same as #2 and #3. Definitely in the past; maybe now, maybe not now.