M.S.
How about the next time they fight, set them in chairs facing each other. They have to sit there, looking at each other until they want to apologize to each other, agree to disagree and let it go, or get to laughing and make up. The 4 year old might be a little young for that, but maybe not, worth a try?
If that doesn't work or they are right back at it, put them in separate rooms. Make sure there are things to do in the toy room/bedroom as well as the living room or whatever rooms you want to use. They have to play alone until they can play together and not fight.
Maybe they need a break. Take them outside, watch home movies, snack time, 30 mintues of tv - whatever can break the cycle for the moment. Of course, hitting can't be tolerated, but sometimes the younger ones know exactly how to punch the older one's buttons. It's hard to fault one over the other.
It's hard when they fight Mine are older now, so there's not the hitting, etc. But, there's a whole lot of bickering!!!!!! I wish I could say that it gets better.................. I REALLY do!!! LOL!
Good luck!!!!