We have a 4 year old child, too. We recently had 2 little foster boys come and stay with us. They were here for a little over a month. They were 4 and 2. My daughter loved having them here - although she did experience some jealousy stuff (she's our only child).
Anyway - when they left - she seemed to have mixed emotions. She went from being sad that they were gone - to glad to have her mommy to herself again.
But - all in all - she bounced back pretty quick. I think the younger toddlers are pretty resilient - and seem to adjust to the changing household pretty quickly.
I've just explained, in very simple toddler-terms, why the children are here - and that it is a good thing for them when they get to return home.
Each child is different, however - so your children may have a different reaction.
But, all in all - I think the younger ones handle the changes easier than older kids.
Best of luck to you.