From what I have been reading in this forum, I would say it is quite normal. If you have been reading other peoples post, you would see. I believe little boys are doing what we (adults) comparative anylasis. You know, "show me yours and I will show you mine". Don't shame him or belittle him for getting naked with his friends, just instruct him and watch him. As for him touching himself he has found that it feels good. That is normal also, but not in public. If you haven't noticed, he probably has erections while doing that. From what I have been reading, even young girls find pleasure in themselves. So, yes. It is normal. I don't believe anyone done anything to him, he most likely discovered it on his own. Just make sure he doesn't carry touching to his friends, even though that would be considered normal also. He found that it feels good to touch himself.
I read the other posts and it does seem pretty normal. I appreciate your thoughts.
It could also be a sign that he has learned it from someone and whilst I agree with the other posts it does seem extreme in a 4 year old and a lot of it, so this does trigger off in my head a red flag of maybe older persons behavior , may be good to be watchful here. Good luck
This is totally normal many parent will walk in on there kids touching themselves but they are just curious it can also happen as a baby to it’s called infant masterbation