I would take the 4 year old to counseling right now, before he gets older and things get worse.( if you can afford it ) He needs to learn to behave and not harm, or bully other children. And not just your child, all children.
Hitting a child, or kids hitting each other, only makes it worse.
Hitting is VIOLENCE, he needs to learn there are other ways to handle anger and frustrations,and HITTING is NOT the answer.
Counseling will help him and you and the family, and the younger he is when he receives it the better !

Sounds like the child has anger issues. If this is not corrected it will only get worse and more dangerous. I agree, you have the repsonibility to protect your daughter and sorry but too bad if DH doesn't like discplining his kid. What is the alternative, let your child become a victim with that mentality. I would get the kid in conseling because it sounds like he has serious control issues. Good luck hon. I know it sucks being in this poistion. My bd5 was hitting my ss8 for annoying her, sometimes even just for looking at her. I finally told him to hit her back and that stopped it. She learned that there are consequences and she didn't like them.

I have 4 brothers two are step. I was the only girl. J my older step brother did this to me. I was shoved several times once ending up getting stitches by my eye. I can tell you that my mother gave us assigned seating at the dinner and J wasn't seated near me. One thing that back fired for them for a little bit was treating me better than him ( I think they were trying to show him they were rewarding good behavior vs bad). One it turned me into a brat for a while and it made him worse. Ultimately they enlisted my older real brother to help look after me and after J had his butt kicked a few times by N things got better. I'm older now and I know why John did what he did. He was hurting. He thought I was taking attention away from him. We ended up being closer a few years later. This is a touchy area. You want one kid to stop so the other doesn't have to live like this, but you don't want to turn the other into a brat. How long have you all been together? I was around 5 when this happened and it only lasted about a year or so. My parents also had to do other things to keep us apart when they couldn't watch us closely. J was in one room playing while I was in another helping my mom with dishes or I was playing and he was with her.

Buttercookie is right. This boy is jealous. He is just a little boy and can't understand that he has to share his dad now. I don't think he needs counseling as much as he needs good old fashioned parenting. You and his dad should sit him down and ask him why he keeps bullying his sister. Listen him. He probably has a lot to say.

BM did this with SD and it worked so well.

She took everything from SD except her bed. She was given chores to do daily without allowance, and given 1 toy back per wk if she did well. If she was naughty or did not complete her chores without complaining she got 1 toy taken back and no new toys that week.

It took over 4 months to get her things back. But we have had very few problems since then.

Try it, it will be hard at first but believe me it works.

Obviously the kid is being rebellious. As I have read in an essay about step-parenting, incidents like these must be dealt with depending on how parents raise their children. But since the kid also stays with his BM who spoils him rotten, maybe it is time to talk it over with the mother and settle once and for all how you both are going to raise him. This will take time though and you may experience resistance from the BM but it should be a team work, raising the kids, that is if you are to live harmoniously and continue living the normal life but have time for other things like personal development and essay writing.