Your upper body's ability to move will absorb all of the energy beyond the amount your brain can easily tolerate. If your head moved 4 inches before stopping, the G forces were no more than you would experience walking on a hard surface. My rough calculation says you may have experienced .5 G's. At most, your neck may be sore.

So, stop the anxiety and move on. I am more concerned about your ability to focus on the driving task.