We're going to be there too! I have a resv. at the Crystal Palace on the 4th for my son's birthday - do you think I should try to change it to an Epcot restaurant?...
...It was hot...really hot. We had lunch with an Imagineer at the Brown Derby and he said he plans his day and how he gets around by which air conditioned buildings he can go through...
...Just be sure to do stuff early and late in the day.
Pooh Pal, I agree. I'd also add that I would not change plans just to avoid crowds at MK. We've done summer and the 4th a number of times. My advice is to assume everything will be busy and plan accordingly. In other words, arrive early, take the mid-day break, and head-out again in the evening. The primary advantage is you'll be out and about when it is cooler and taking a break to cool-off at the pool or avoiding the afternoon thunderboomers.
At this time of year, it is always really hot and humid. I've always lived in places where it is really hot in the summer, so the heat is not a big deal to us. If you're not acclimated, however, my advice is to take it nice and easy (i.e., no commando touring in the summer). Finally, with respect to crowds in other parks on the 4th, we once went to Epcot for Illuminations on the 4th and the closest we could get to Showcase Lagoon was about half-way down the Odyssey Center Pathway.