My one year old neutered male cat plays with our 4 week old male abandoned kitten?

Sounds like you're doing a spectacular job. It's great that you're supervising their play and you should probably continue to keep an eye on them until the kitten gets a little stronger, just to be safe. I doubt your older cat's behavior is aggressive. You would be able to tell if he was actually upset and trying to injure the kitten. More likely they are just play fighting and having fun. I haven't had two cats concurrently but it sounds a lot like playtime between my cat and dog. They're best friends but look like they're trying to kill each other when they play. It may look rough but if your cat was hurting the kitten the kitten would squeal and clearly express that it was in pain, letting your cat know to stop whatever he's doing. If the kitten was being hurt he also wouldn't come back to be further attacked. If your cats sleep together they obviously enjoy each others company, which is great. I'm glad the kitten seems to fit in so well as a member of your family. Thank you and your daughter for helping a stray. Best of luck with your kitten! :)