I went to WDW last year from 6/26-7/5. I'm sorry to say, I will never go this time of year again... the crowds were typical when we first got there, but within a couple days of the 4th and things got out of control. They raved about how great the fireworks were going to be, and they were good, but nothing better than usual. We were sitting right near the castle, by the tomorrowland entrance... and it took a good hour to get out of the park at the end! The weather that week was typical, a few short thunderstorms. A few nights though the rain continued through the night. Not heavy though.
My suggestion- don't go in a park on the 4th. Relax by the pool, go to DTD for some shopping, or maybe resort hopping. But, in my opinion, it's just not worth going into a park on 4th of July. I've been going to WDW in July or August most of my life, so I'm used to crowds. But this was insane!