:) Magic Kingdom is my park.
If you go on a light crowd day, one day at Magic Kingdom may be enough. Here is a site that lists anticipated crowds: Orlando Crowd Calendar
Keep in mind, a 4yr old will start the day very enthusiastic but will eventually fall asleep :)
Also, FastPass planning is very important to fully experiencing MK. The sooner you buy passes or make hotel reservations (at a Disney hotel) to sooner you can make get FastPasses. If you do stay at a Disney Hotel, you definitely want to take advantage of the Extra Magic Hours benefit.
How long you want to dedicate to the Magic Kingdom depends on how long you will have at Walt Disney World. If you have 3 days, I would recommend 1) Magic Kingdom, 2) Disney's Animal Kingdom, 3) Disney's Hollywood Studios* and/or Magic Kingdom.
*Disney's Hollywood Studios is undergoing major construction right now so you won't necessarily need a full day.
Finally, EPCOT is not a 4yr old's park, with one possibly HUGE exception: Frozen Ever After.
+1 for The Hoop-Dee-Doo Musical Revue. We go every year.