I would highly reccommend enrolling a 4-year old in a MA program, just be sure that the program works specifically with smaller kids, and isnt just a "kids (1-17 yrs old)" class. When a child begins learning younger, they develop instinctive basics, and they benefit later on, especially if they are continuing MA. Just dont expect your son to be good to begin with, or even in a year- 4 years old is still quite small, and it will probably take a good amount of time to get the basics, but it will be worth it in the end