How can I get them to clean their rooms?
More tips on getting him to pick up the toys in his room would be helpful as well.
It's too much to expect. My daughter is 12 and is finally showing signs that she can look at a mess and not be overwhelmed by it. What I do when she is reacting in ways that don't make sense to me is I remember what it feels like when the house is totally trashed and I don't know where to begin and I despair that it will ever be neat. That's what it feels like to a little kid, even when faced with a little mess.
If we see the environment we provide for kids as our gift to them, then it's our responsibility to create that environment for them, not theirs. See the clean up as yours. And ask him to help. (But accept that no is an acceptable answer since it is a question.) Ask if he'd like to be with you to keep you company, or to listen to a story tape with you while you clean up.
They will help when they see that their help -- even what seems like a token that's far beneath what they're capable of -- is appreciated. They will help when they know that they don't have to help, when they know that you recognize that the tasks belong to you and that they are helping not being conscripted onto a chain gang merely because they happened to be born. They will help when they're mentally and physically capable of helping. (4 is way too young. Even 11 was too young for my daughter. But at 12 she is very helpful because her brain had developed enough to be able to process what needs done and how she can go about doing it.) They will help when we respect that what they're doing is important to them and trust that they would help if they could.
When he starts to resist, it isn't a fun game anymore.
Try turning it into a game. He doesn't have to participate but he might if it looked like fun. If you have bins, you can throw things into them. You can put on lively music and see how much you can pick up before the song is done.