You issue should not be worrying about the hit to your head.

You should be considering whether she puts your life in danger with her domestic violence acts, especially while you are driving a car. There are many objects that can stop your car quite quickly with high G decelerations.

At 40 mph, you travel 60 feet per second. Oncoming cars can be 1/2 second away. 80 mph head on impact speed would be devastating. 40 mph into a pole would be 80 Gs.

A slap to the back of the head is not 4 Gs. There is no way to calculate such a hit.

This concept of trying to score the damage from any contact to your head will destroy your life. What matter does it make? It already happened. You can do nothing to reverse time.

You can make decisions about any future risk of her hitting you.