Honey, I really hate to say it, but he has been exposed to *something.* This is pretty specific behavior for 4. It's one thing for a 4-year-old to think saying the word "penis" is screamingly hilarious because it is sort of verboten; that would be normal. But grabbing, continuously manipulating, propositioning and licking are not normal for 4. Are you certain he has not seen you and your boyfriend being sexual? If there is absolutely no way in the world, then I would wonder who has been doing or showing him something.
From a guys point of view, this is natural. I'm raising my 4 boys, a 13 year old, a 6 year old and twin 3 year olds. I have caught the 6 year old doing stuff like that also have caught my lil bro who is the 13 year old doing stuff with his friends.My sons are always playing with their penises, its there, its going to get played with. Try to teach him privacy. It is hard for them to understand what that is at such young ages. As for trying to expose other kids, give him time outs. Sexual exploration is normal. A lot of times kids learn this stuff from their friends, once they know it feels good they keep doing it. I honestly wouldn't worry about him being molested if ur sure he hasn't been. Ask him where he's learned this behavior and don't get mad at him for doing it. If u yell it could make things worse. Sit him down and talk to him. O also on the privacy note, to help explain it to him, if he's playing with it send him to his room and explain that this where he can play with his pee pee and only in that room. My sons like to pull their pants down and show me their penises too. Just don't acknowledge it. Act like u don't even see it. What he is doing is trying to get a reaction out of u. Good luck.