Hi,
I was in the Magic Kingdom this past July 4th.....We always go in the busy season, but WOW! was it packed out! We just had to have the attitude that you wouldn't be able to do everything that day. We took advantage of the fast passes (although I don't think there were any left by early afternoon) and just relaxed and did what we could.
However, the 4th fireworks at the MK were Awesome! Just make sure you stake out a comfy place to watch them VERY early on and just have a snack while you wait. You don't want to be all the other people just walking back and forth because they can't find anywhere to stand!