4 Year Old Being Aggressive

I would watch the older sister very closely because she very well may be provoking him to get him in trouble. My son is 2 yrs older than my daughter. Both good kids, but come on - they ARE brother and sister! He would tease her when they were younger and I would tell him not to obviously, but I would also tell him that he better watch out, cause once she gets bigger she's not going to take it anymore!!! The younger one doesn't have the verbal skills as the older one or the emotional control, so if they act out physically then they are perceived as the "bad" one at the instance, while the more silent acts of teasing etc from the older one go unoticed. And when they get punished and they other sibling does not, it just adds to the frustration which can get aimed at you (as the "punisher"). Not being physical doesn't necessarily mean "gentle hearted". Girls can be very smart and sneaky........Just trying to speak from experience - my kids are girl 7 and boy 9. Best friends and worst enemies!!!!!

I am dealing with the same thing except there are 3 years between my son and daughter. Time outs don't seem to be very effective. Usually what I do is send him to his room to calm down. I also get in between them when I catch it escalating, picking him up to get space between them if necessary. I just posted a question about using martial arts classes to help him learn how to control himself and I think I'm going to look into signing him up. If you look for that question, there were some other good suggestions in the answers.

Sounds like you're doing all the right things. You may have to put her in time out if she is really aggravating him. How long is he in time out? four minutes? maybe add a minute for the second offense of the day. etc.
Another thing you could try is "magical" bracelets, anklets that will remind him to keep his hands to his self. Turn it into a more positive thing
Remember to keep hands and feet to yourself (instead of NO hitting, No punching) and a sticker chart (one sticker for each morning, afternoon and evening of keeping hands and feet to self.

I am thinking he needs more physical outlets, exercise?

I would also put both kids on a timeout so you aren't favoring one child. I would talk about hitting when he is calm and at another time. Has he seen other people hitting?

Do you have any time alone with your son? Maybe he needs some special time?

Good luck. Hope you see some changes.

You got some good advice, I just wanted to add Siblings without rivalry. People have said that is a great book to solve the sibs fighting.

If time outs aren't working think of what would affect him (video game, etc) and take it away for a certain amount of time. When he is calm teach him how to breathe and calm down or stomp his feet. On her side, she needs time-out or something of sorts for agitating him too.

Have you tried spanking? My girlfriend's son threw a fit in a store and she took him to the rest room and spanked his behind and again when they got home. He never again did it. It's worth a try.