I have been thinking about posting a very similar question... My boys are 2 and 3-1/2 and have always shared a room. My 2yr old is still in his crib, but we need to transition him to a bed b/c #3 is due in March. Anyway, my problems of late are more with my older son. He's in bed around 8:30 and, like you, is often still awake at 9:30-10. Last night, he was still awake at 10:15! It's so frustrating, and I also feel bad for him because we need to wake him up by 7a at the latest to get on the road for daycare/preschool and work for me. He's either constantly out of his room or yelling for one of us to come in the room for something. We generally end up threatening to take away toys for a day - sometimes this works. It keeps him from getting out of bed, but doesn't necessarily help him fall asleep any sooner.
I have been wondering if he's ready to give up his nap so that he'll be tired enough to fall asleep shortly after getting in bed. I'm not sure he's ready for that, though, because he'll still nap for 1.5hrs. If he doesn't take a nap, he ends up falling asleep around 5p. If this happens, you can imagine how hard it is getting him back to bed at 8:30.
You didn't mention if your 4yr old is still napping. Maybe that would help him fall asleep sooner? Sorry I don't have much to offer. I'll be checking the responses you get for any insight, too.