My 4th old acting out

My son is 4yrs Old and for some reason is acting out sexually. He and his three ye old sister went to the bathroom and when she bent over to Pull her panties down he walked behind her and pushed his penis against her but. As soon as saw this I made him move away from her an asked where he got that from he said he saw it on his uncle tv. His uncle denies it but I don't think he would have came up with that on his own. Today his sister told me he touched her but, what should I do he doesn't  really understand y it's wrong an I don't wanna get into detail with him I think he to young but I really don't know what the next step should be.

[Kevin Kennedy, Ph.D.]

Parents often make this more difficult than it needs to be. He does not have to understand anything about sexual behavior. You simply tell him that he may not touch other people in the area of their genitals and buttocks and he may not permit others to touch him. Set a very firm limit about this. Young children will copy what they see. You should tell his uncle about what happened and, if you can't trust that he will be supervised at his uncle's home and that good judgment won't be used about what your son experiences, you'll have to decide whether he should be over there without you.