D.M.
He is most likely becoming more aware of that part of his body which I guess is normal. I would sit him down and see if he can tell you why he is doing these things, just after he does them. I would explain that his private parts are private, that if he wants to 'study' them that he should do it in the privacy of his bath or bed. Touching your breasts are not acceptable, they are YOUR private parts. Kissing he sees on TV is more make believe than anything else and is NOT appropriate for kissing his mom. Show him what a mom kiss is.
But, try to have these talks when you 'catch' him in the act. Find out if he has any questions about his privates and then answer them as best you can as is appropriate for a 4 yr old. It is most likely just a new found awareness and you and your hubby will need to explain appropriate behavior with that awareness. Do be sure to not make it 'bad' behavior, just a matter of when and where that is appropriate. Good luck! D.