Yeah, I think four is fine. Our four year old is quite happy and confident round the pets and although we no longer have a hamster ( died a couple of years ago) we do have guinea pigs which he loves and are delightful. We've never found hamsters bite either, and if your kids won't clean out the cage of an animal belonging to them and their responsibility and regulalrly feed them, then there's nothing wrong with the pet , or the child, just the parenting,sorry matt. They need to be taught that they have responsibilities which can't be abdicated, one of the whole erasons for having a pet, otherwise it's pointless and just gives them a negative messae instead of a positive one.