Why does your two year old take so long to get off to sleep? Perhaps you should address those issues first, getting her into a good sleep routine so she is asleep when you put the 4 year old down. I sympathise with your son - he is old enough to think that he should be able to stay up later than his little sister. If you don't know how to set up a sleep routine, get books from the library or use the internet and then persevere, persevere, persevere. It won't come right in one or two nights!