Advice PLEASE

Hello everyone! I had some concerns and just wanted some input. If anyone had gone through anything like this some support would be amazing. I have a 4 year old step son. He has become very aggressive and is constantly hurting his 3 year old brother. We only have him every other weekend. Me and my husband are not exactly sure what goes on at his mother's. We do know she recently had 2 pfas towards her husband the last one got physically. He supposedly witness it. We have taken all electronics from him. He is obsessed with sonic and huggie wuggie and exe sonic. They are such scary characters and condone violence. He says he watches it at mommys house. We cant allow him YouTube or anything electronics because he is watching violent things. My 3 year old now is now scared of him and puts his hands up in defense if 4 year old comes near. He has hurt him out of no where and he just can't keep his hands to himself. He shoved him off a chair today and my son hit his head extreamlu hard. I can't allow them to be alone anymore or even play together I am terrified he is going to seriously hurt him, another child, or an animal. Has anyone had any experience or any advice on what to do for a aggressive 4 year old?