We too will be there for the July 4th holiday, our 4th time. There are more fireworks on the 4th, but that is it. I can tell you that during peak times, like in July, the hours are longer. If I remember correctly MK was open until 11pm the entire time we were there(first week of July) so it's great to visit early, then leave during the busy times, then head back late in the day when it is less crowded. The fast passes will run out about half way through the day on really busy days so be prepared. Mission Space and Test Track seem to be the ones that go the quickest, so if you want to ride those hit Epcot early, and then go back later in the day when your fast pass time rolls around. Hope this helps!
Laura