First, let me say that the MK is insanely crowded on the 4th, and I would never attempt to be there that day. However, my family and I were at Epcot last 4th, and I actually just got done posting over on the restaurants board in response to someone's question about where to eat on the 4th of July - I'll paste that same response here:
This year we made a last minute decision to go to WDW for the 4th. We decided on Epcot for the day, since we believe it handles crowds better than the other parks. We got a room at the Dolphin for the night of the 4th only, because we wanted to watch the Epcot fireworks and then be able to walk back to our room, avoiding the huge crowds on the bus/tram/parking lot. We thought there was no way we would get an ADR (this was about 3 days before!), but when I called, I had my choice of several World Showcase restaurants for lunch (I didn't even try for dinner). We chose the San Angel Inn.
Our plan worked great - we enjoyed the day, even went back to the Dolphin for a swim in the afternoon, had counter service that evening at the food court at the Land, staked out a great spot for the fireworks about 90 minutes before time, and afterwards, just strolled back to the Dolphin, where we enjoyed a fabulous night's sleep on the "heavenly beds".
So, based on our experience, I can recommend: Epcot for the day of the 4th (handles big crowds well), a World Showcase restaurant for lunch and a counter service dinner (saves time in the evening, freeing you up to stake out your fireworks spot), *definitely* Epcot for the fireworks (they were amazing!!! and after all, you've gotta have your fireworks for the 4th), and staying at the Swan or Dolphin or any other Epcot area resort, so you can get back to your room after the fireworks without dealing with the incredible crowds on the transportation system or roads.