Hi Abby,
With four year olds, I would spend a full two days in the Magic Kingdom. The remaining two days, I would break up this way:
Animal Kingdom - My children love animals! This was especially true when they were in preschool. You could very easily spend a full day at that park. The Maharajah Jungle Trek and Pangani Forest Exploration Trail give you the opportunity explore and enjoy animals at your own pace. Be sure to do the Kilimanjaro Safari as well. If you find that you have finished everything that you want to see in the Animal Kingdom, head over to Epcot for the remainder of your day.
Epcot - As wonderful as this park is, there isn't an abundance of attractions lot for preschoolers. Head to The Seas with Nemo and Friends. This slow moving, mesmerizing ride ends in a multi-level aquarium. Your little ones will love watching all of the various fish, dolphins, sharks and more. The Imagination Pavilion is another great stop for them. They will also enjoy riding Spaceship Earth. The majority of the World Showcase may be something that you want to save for when they are older.
Hollywood Studios - There are several shows here that are great for preschoolers. First on the list is
Disney Junior - Live on Stage. You'll also want to experience Toy Story Mania. Depending on if you have boys or girls, you might want to sign up for the Jedi Training Academy, watch Beauty & the Beast, Voyage of the Little Mermaid and Muppets Vision 3D. With the remainder of this day, you could also head to Epcot on the ferry boat to finish up any attractions you didn't get to on your Animal Kingdom day.
I hope this helps. Please let me know if I can assist you further,
Kaylene