Now I would try to have him determine if he has a dominant eye yet (which he probably does). Then build th bow to match the eye dominance. It will be easier to learn to shoot left handed if he is cross dominant than change later in my opinion. Having been trained in the NASP program they really emphasize the eye dominance, because it helps the student become more consistent, and feel successful quicker. I realize at four he is not going to be super serious and the point is to have fun. But I think if he starts building form and muscle memory with the side that matches his eye dominance he will improve quicker as he ages and starts to work to be more successful.