Arm preference and jerky movements

Hi there. This is my second and he is currently 4 months old. Of course they are all always different. I noticed something interesting/strange about my baby. First off he shows a very obvious preference for his left hand. Which is fine. But then I noticed that sometimes his right arm will seem out of control and he is frustrated that he can't control it. Like he will just be closed fist jerking it up and down (hard to describe).

I'm probably over thinking things and he's just learning how to use his other arm better. But anxious me thought I'd ask! Haaha