Why is your 4 year old son is blinking excessively?

It is possible that he has genetic cataracts, or just something in his eye. Check for obstructions, eyelashes, and also ask him if his eye hurts, is cloudy, or, and this sounds silly, I know, but if ask if he's doing it because a friend told him. If none of these things are correct, then I suggest visiting an eye doctor, a.k.a. and ophthalmologist.