4 year old humping 3 year old?

We have had our 4 1/2 year old for almost a year. My husband caught her dry humping our 3 year old in a chair while they were watching a movie. He had litterally walked out of the room for 2 minutes and came back and she was going at it against the 3 year olds back. He said they were laughing but he made her get off and called me. I got home about 3 minutes later and when I asked her why she did it (we have had talks about this in the past) she said because she knew I wasn't home. My husband watches them very carefully and she knows that he disapproves of this behavior too. When I have caught her masturbating in the past (about 6 times) have been calm and explained that we do this in private and to NEVER do it to someone else. Especially the 3 year old. This time I lost it and screamed at her and sent her to her room. We had to move her out of her beautiful newly painted room to a spare room by herself. I reported it to the cw and no one seemed to think it was a big deal. I have been reading like crazy about this and can't seem to feel any better about it. I'm not a prude but was highly sexaulized as a child and it distracted me so much from what was really important. How do I keep her from becoming like this? I turned out OK I guess but I can't help from feeling disgusted by her. I know that is an awful thing to say about a 4 year old and I hate feeling this way. Any one have any ideas? We are starting up counseling again. I might go just for my own issues too. I don't want her continuing to do this to others. Read my previous posts if you would like more details. Thanks.