I think it is important that he understand that his little brother is not "making him mad", but that he is choosing to get mad. I am having the same trouble with my 4 yr old hitting her younger brother. I ask her 3 questions when she does it. 1. What did you do wrong? If she tries saying, "he started it", or something like that, I tell her I'll talk to him in a minute. I want her to own what she did, not play the "blame game". 2. What are you going to do different next time? This way, I know she knows the right thing to do. The last is not really a question, 3. Go and try it. And of course, she has some kind of puishment because I will not allow her to physically hurt her brother. Most of the time she wants his attention, so I dont let her play with him for a while.