When they all got in the coop, he charged at her. I don't want anything to happen to her. She looks so scared. Yes, he is practicing mating and will get the hang of it soon. But 4 months old pullets are too young, and indeed he can damage them.
Young cockerels are often quite aggressive.
And if you interfere with him, he may become aggressive with you. I probably will get rid of him. It seems only one hen is being outed. The rest hang around him and have no problem. I guess along the way the neighbors will get sick of him.