They are all about 4 months old.
There is a world of difference in immature cockerels and mature roosters' behaviors, that's a big part of what makes this so difficult at 4 months. The cockerel's hormones are telling him "Dominate! Dominate! Dominate!" He does that by mating with them or just bullying them. The pullets are not mature enough to know what is going on but they know they don't want to be dominated so they resist. What you are describing is pretty typical behavior between immature cockerels and pullets. What normally happens is that when the cockerel matures into a rooster and the pullets mature into hens you get a really peaceful flock. But getting to that point can be hard to watch. I don't consider that aggressive, I consider it a phase they are going through. Not all cockerels or pullets mature into good roosters and hens but most do, as far as behaviors between each other. But sometimes you get a rooster or hen that is just a jerk.
Sometimes when you have two or more cockerels they fight each other, usually not a lot but sometimes a lot. When one establishes his dominance the others may take a submissive role. The dominant "flock leader" has certain rights, privileges, and responsibilities. The submissive ones have those behaviors squashed, the dominant one often won't tolerate them assuming any privileges. Sometimes all the cockerels will try mating the pullets, sometimes that is the privilege of the dominant one. I'm not sure if your two have become submissive or if they are just late maturing and the hormones haven't really hit yet.
I don't have any foolproof suggestions for you, sometimes I get it wrong. Typically the pullets mature enough to act like hens when they start to lay. With cockerels it's harder. I had a cockerel mature enough at 5 months, though that is rare. I had one wait until he was 11 months to mature enough. Mine typically flip that switch and gain control of their hormones at 7 to 8 months, but it can really vary by cockerel.
What I want to see is that the male dances for the girl he is interested in and she squats. I'm OK with her initially running away and him chasing, as long as she squats when caught without him getting very physical. She's not really resisting him, just wanted to see if he was really interested. This is when they mature, not still immature cockerels and pullets. But if you do see this with cockerels and pullets, it is good. It shows they are growing up.
The head grab is an essential part of mating. The male grabs the back of her head. That is the signal for her to raise her tail up out of the way so he can hit the target. That's not him being mean and brutal, it's essential. Immature cockerels sometimes have really bad technique with this, as they mature they typically do much better. If the back of the head becomes bare there is a risk he could cut her skin when he grabs back there. So pay attention to that.
This is when mating. If he is grabbing her head or back a other times, he is bullying. As long as no one is getting hurt I'm OK with this, it's part of them growing up. But if you see blood or bald spots you need to intervene.