Amelia:
 
Assuming he is otherwise a well 4 year old, try swimming at a YM-YWCA or a community pool.  Non weight bearing exercise like that would help.  Climbing and running helps.  If you live in an apartment building and the stairs are safe with you present, climb them. Limit TV and game time and keep moving.  Biking and games are sometimes helpful as well.
 
However, be sure that his medical home or pediatrician has examined him recently to be sure he is capable of the exercise.
 
Wayne A. Yankus, MD, FAAP
expert panelist: pediatrics