MK fireworks in past years have been presented on the 3rd and 4th. While the park is busy on both days....it is PACKED on the 4th, as are all of the parks.
MGM does a special fireworks display on the 4th also.
Epcot has a 4th of July tag on the end of Illuminations which is very cool. Epcot usually has some extra "patriotic" entertainment in the park too, usually in front of the American Adventure.
LY Pleasure Island had a nice 4th of July fireworks show, but as they are no longer doing nightly fireworks, and PI is under much construction, I would not could on it this year.
You best bet, see MK's on the 3rd, and MGM or Epcot on the 4th. It is possible to see both MGM's and Epcot's on the 4th, as there is about a 30 minute gap between the 2 shows, and if you walk fast, you can make it between the two parks with time to spare.