4 yr old exhibiting sexual behavior?

My 4 yr old son seems to be exhibiting sexual behavior that I feel is inappropriate. I know masturbation in children of this age is normal as they are "exploring" themselves but I have caught my 7 yr old female cousin trying to touch his private area. Also, my husbands niece is a year younger thann my son and seems to always want to "play" under the covers and try to make my son lay on her. Today, my brother in law caught the kids in my sons room and my son had his pants down, he tried to pull them up when he saw his uncle so he obviously knew this was wrong. I'd also like to add that my son has recently started wetting the bed very frequently and just seems to want to focus on sexual behavior that may or may not be normal but I am very uncomfortable with it. My son also seems to act sexually inappropriate toward my husband (ex: he tries to sit on his foot and rub himsellf against my husbands foot ) I am a stay at home mom but am not sure if my son may have possibly been abused. Is he too young for therapy?