As everyone already said, look for an underlying reason saddle,bit,feet... Then pay attention to when it happens and although no one wants to admit to being an average rider, pay attention to you. Does he do it when turning a certain way,only on transitions,in a ring or on a hack. Does he do it with every rider or only certain riders. If the riders balance is slightly off the horse will imitate the rider, so if you sit hard on transitions because of balance his head will go up, if the riders legs[thighs] are tight against the saddle to try to keeo from falling off the horse will get agitated and fuss with his head. If a great rider it could be nervousness/apprehension. You say the horse is just a 4 yr old. If the rider is worried about what may happen[ its windy,the dogs out,he didn't eat yet,he doesn't like yellow...] He will pick up on that and not have a calm ride either. Hope you figure it out. Good luck