I don't think it's necessary to wean early, but it would be fine to wean at 4 weeks if you choose. Just offer lots of hay and limited pellets to begin with.
If a 4 week old is mounting other rabbits, it would more than likely be a dominance behavior rather than a breeding behavior. This could be learned from the doe, so keep an eye on her to make sure she isn't "bullying" the litter. Some does do get tired of their kits around this age and start mounting them. If you notice that happening, they should be weaned immediately.