My 4 year old pees on the toilet fine but becomes hysterical if you try to get him to poop on it. At what age is this behavior no longer "normal".

Painful experience. No history mentioned for this child previous experience on his bowel movement , bad /pain or stressful events/events or toilet training. if he has underline problem such as constipation or painful bm that has to be addressed and then retraining with unhurried supportive manner should be followed. your child should be seen with his pmd first and then go from there.
Poop. Your child may have a problem passing a hard stool and now he associates going to the bathroom with pain. He will need to be retrained. Speak to your doctor or a pediatric gastroenterologist on how to do it.