4 year old boys getting naked

My 4 year old son has been caught several times getting naked with his guy friends (all the same age).  It happened 2 or 3 times with 2 different friends about 3-4 months ago.  We spoke to him (as did the other parents) about how his penis was private and he was not to show it to others.  During these dicussions it also came out that my son had tried to "bite" his buddy's penis one time, and another time they said they were touching bums (like bum to bum)!  After many firm discussions and them not being allowed to play in his bedroom for a long time there had been no more problems.  Until yesterday when he was caught upstairs with a friend naked, they had apparently both gone pee and then they were getting changed.  Then today he was naked with another friend.  When we asked him after about it he said they wanted to measure them!   So is this normal?  We've talked to him so many times, he knows the rule "no getting naked". Otherwise he is a super little boy but we just are at a loss for what to do about this behaviour.

On a similar subject our son also likes to touch his penis (by himself) when he's watching TV.  He lays on his hands and touches himself and gets all red in the face and sweaty.  Is this normal at 4 years old (he's done it for a few years)?  We've talked to him about how this is a private thing to do and he should do it by himself in his bedroom.  If these are not normal behaviours what suggestions do you have?  Should we go talk to anyone about this?
Any help is greatly appreciated

[Liquid4444]

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.

[Samantha88]

Thanks Liquid4444
I read the other posts and it does seem pretty normal.  I appreciate your thoughts.
Samantha88

[Liquid4444]

You are welcome. Glad I could help. I am on Yahoo.

[margypops]

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