I do everything right-handed, even rifle hunting. I figured out I was left-eye dominant when I started shooting clays. I took up archery hunting 3 years ago at age 30. Since I had never shot a bow I really didn't have any bad habits to break other than getting used to shooting lefty. It wasn't that hard and I know I'm a better shot left handed. I know 4-year olds can be difficult but I'd see about switching him. Try not to make a big deal about it. Try to sell it that he'll be a better shot and being able to do things with both hands is a good thing.