Daughter being aggressive with son

My almost 4 year old keeps being aggressive to my 10 month old and idk how to stop it. He’s usually in his playpen but she’ll go and push against it if he’s near to push against his head. A lot of times she’ll smack him on the head, sometimes repeatedly. I tell her to use gentle hands and if she doesn’t listen i’ll remove her away from him but she laughs and goss back. She didn’t do this when he was smaller so idk what’s going on. I’m just tired of her hitting him😭