We visited with my daughter at 3, 5, and 6 (skipped Disney when she was 4, the year my son was born), so I'll use what we did with her as a gauge. Ranking the 4 parks for a kid around 4 years old, I would say goes like this:
Magic Kingdom has a lot of pre-schooler friendly rides and events. For example, It's a Small World, Peter Pan's Flight, Pooh's Playful Spot, Toontown Fair and Fantasyland are all pre-schooler friendly.
Animal Kingdom is cool, simply because most pre-schoolers just love seeing the animals. Plus, the Kilimanjaro Safari, The Boneyard, and DinoLand USA were favorites for my daughter each time.
Epcot has Seas with Nemo and Friends, and Kidcot Fun Stops at the World Showcase, but not a ton of pre-schooler friendly rides (some have height restrictions).
Hollywood Studios has Playhouse Disney-Live on Stage, and a great show in Voyage of the Little Mermaid, but it's smaller compared to the other parks (at least, we always seem to get through it quicker), and not too many pre-schooler attractions (daughter never really liked Muppet Vision 3-D).
If you rent a double stroller or bring your own, we were able to "rough" it out with our kids staying at some of the parks all day. If you start in the morning, you can do each park a day with the exception of Magic Kingdom, which generally takes 2 days to get through.
If you're staying at a resort, however, it doesn't hurt leaving the parks, then coming back later in the evening. We had at least one meltdown with our 2 year old and the only solution was to get out the environment (too much stimulation :-)
Good luck on your disney planning!