Epcot has tons to do for young kids! My DD was a little over 4 on our last trip and loves Epcot!
First, there are lots of rides and attractions for little ones - The Nemo ride and Turtle talk with Crush, all of the activities in Innoventions, Living with the Land, Journey into Imagination, your 4 year old might be tall enough for Soarin and Test Track.
My DD loved the Kidcot stops which are in Futureworld and World Showcase and they are free! In Future World, you make and attach various objects to a lanyard. In World Showcase, each country has a symbol that you attach to a mask. Also, we bought the passport for WS and felt it was worth every penny (about $11). She loved getting it signed and stamped in every country. Each CM also wrote a note to DD in his or her language with many of them writing her name as well. She then adhered the stamps that came with the passport after going to each country.
This link is to Brad K's guide to Kidcot -
http://www.stitchkingdom.com/kidcot/
Another thing many people don't realize about Epcot is how many characters you can find there. The lines are usually a little shorter than MK and you can sometimes find those harder to find characters. My DD loves meeting the characters and getting their autographs - she loved this as well on our first trip at age 3 1/2. In Epcot, we met Jasmine, Aladdin and Genie, Mulan and Mushu, Mary Poppins, Alice, Marie from Aristocats, Pooh and Tigger, etc. I used the Steve Soares site listed above to look for character spots and times as well as entertainment schedules.
After MK, we spent our most time in Epcot. Enjoy!