My four year old is portraying sexual acts.

I just found my four year old son and my three year old daughter in the room talking about putting her butt on his lap. His pepee was hard, but they wheren't naked. They haven't been sexually abused I am the only care giver they have had. Could it be from them watching t.v.? I am really upset and don't know what to do!!! My oldest told me they learned it from their friend which they call wiggling. I don't know if he has been abused or not. This isn't the first time I have caught them doing this, with their clothes off before. Should I get onto them about this or just tell them we don't do this? I have told my friend about her son teaching my children this and she doesn't think he is being abused. I don't know what steps she has taken to find out if he has or not. Is this signs of sexual abuse or are they getting it from the t.v.? I am really upset and thinking of taking them to a therapist or something!!! Can someone please tell me if this is normal or not!!!!!