My son has sensory integration dysfunction and when he was a little guy even younger than 4 years old, it was a nightmare to wash his hair. He also was not afraid of water but really hated the hair washing thing. It was a sensory issue. For him, the head rubbing was actually hurting him, it was painful for him. And so he created a fuss.

Someone mentioned it earlier - the child will not die if the hair is not washed. We settled for just a tub bath. Water could be splashed about - but not in the face and not on the head. We got him to lay down in the tub with just enough water to come up to his ears - and make water angels. The little bit of water flow got the hair wet and since it was a soapy bath, the hair got some cleaning by accident. Using force was not helpful and only made the situation worse.

Intervention for sensory integration dysfunction is occupational therapy which helped our son out a lot. If you think maybe your son is having a sensory moment - I would suggest the book "the out of sync child" which was really helpful for us.

My son is 11 now and swims like a fish, washes his own hair without a fuss. It all worked out.

Best of luck