4 year old son says hurting people makes him happy.

My 4 year old son has been hitting his sister's lately. We've talked to him and told him it's not okay, and asked him if he would want someone to hit him. He always says no. We've also asked him why he did it and he always says he wants to. Well today his sister was sitting on the floor watching tv and he was sitting on the couch right behind her and kicked her in the back. I sent him to his bed then went in there to talk to him about it. I retold him that it isn't okay to hurt other people and that it needs to stop, asked him why he did it and he gave the same "I wanted to" answer. However after that I asked how it makes him feel when he hurts other people and he responded with "happy". I am at a loss of thought about this... what should I do about this? How should I approach this? via /r/Parenting http://ift.tt/2z8rWLw