MK & EP both do a very special fireworks on the 4th of July. As pp said, MK does shoot off the same fireworks on the 3rd. Crowds will be fierce at MK on the 4th. DHS also features a fireworks display on the 4th of July. This fireworks display features modern music and is very similar to the fireworks in my hometown.
All 3 fireworks displays are on youtube.
I would personally go for the MK on the 3rd and then head to Epcot for the 4th.
I hear that EP is much more manageable on the 4th of July.