We always go in July. Yes it's hot and humid and you can count on some quick after thunderstorms. That being said we love going in July. The parks are open late, the parades and fireworks are scheduled for every day, sometimes twice in one night! Here's the way we keep sane. We get to the park at opening, stay until around lunch time, go back to the resort for swim/nap break, go back to the park (or nearby resort) for dinner and do the night time activities (Fantasmic, Illuminations, Spectro, etc.).
All three of the parks have special fireworks on the 4th. I'd say just avoid MK on the 4th and you should be fine. We usually go to EPCOT and I've heard people say that AK and MGM were not too bad on the 4th. Have a wonderful trip - we'll be there for the 4th also!