I am a personal trainer in a fitness club. I don't recommend four-year-olds to do strength exercises. Because he is too young, the bones and muscles of all parts of the body are in the growth and development period, which is extremely delicate. Whether it is weight exercise or resistance exercise, in the long run, it will have a bad influence on their future growth. You can do some strength exercises when your child is seven or eight years old or even later. There is really no need to practice too much anaerobic strength training now. Children can be allowed to do some moderate aerobic exercise, such as running and skipping rope. Most fitness coaches don't recommend strength exercises for children aged three or four.