my 4 yr old boy is being innappropriate

[racdryan]

My 4 yr old boy seems to like "boobies" he points them out at the store in magazines, on tv (fully covered) etc.  (and I never breast fed him) Tonight while playing with my 9 yr old boy and my best friends 3 and 11 yr old girls who they have grown up together with, he tried to touch/rub the 11 yr old's boobies with a back scratcher and thought it was funny.  He also often while at home will pull his pants down and flash us and laugh.  I NEVER had this problem with my older son and have always taught my boys about their "private" parts and what is considered innappropriate.  I am humiliated that he seems to think this is so funny and so afraid that when he starts Kindergarten this fall, he will do something and then be labeled as the "naughty boy" in class. My husband and I have both sat down with my 4 (almost 5) yr old and tried putting it in a way that he would understand that this is not ok behavior.  Any suggestions?  

[margypops]

Could be learned behavior would he ahve seen any 'girlie' magazines, i think it would be a good idea to quietly watch the dynamics when he is with the older children as it sounds to me as if he is copying, and thats what older siblings do sometimes...Be aware