Since July 4th is on a Monday this year, expect some pretty heavy crowds that whole weekend leading up to the holiday.
Also, we're int he rainy season here in central Florida, so keep an eye on the weather forecast. It rains almost every day now through the middle of September. You can pretty much set your watch to the rain storms. Sometimes it'll rain for an hour or less... other times it'll rain all day. But when it rains, people chicken out and go back to hide at the hotels and the parks are pretty empty.
The best thing to learn about to help you with the crowds is how to use fastpass. If you've been to DL, you'll know. But for newbies, it's the one thing you'll want to know about. It will save you from standing in long lines for major attractions. It's free with your ticket. You insert your ticket in the fastpass machine located near the ride, and it will issue you a fastpass with a return time. It will have a 'window' of an hour it is valid for, but will almost always be honored all day after the return time.
Nineteen years ago, my family's very first visit to WDW took place during the Fourth of July holiday. Prior to that, my parents had only visited during the off-season in the mid 80s so they didn't realize just how packed the resort could get. That was the last time my family even thought about visiting WDW during a major holiday, ESPECIALLY a summer holiday like 4th of July. Just heed the advice above about the daily rain showers (afterwards they make for some good rainbow shots though), stay hydrated, be patient and have fun! Most of all, don't miss the fireworks!
I work in a jewelry vault. There's a swimming pool filled with jewelry and coins, and I pretend I'm Scrooge McDuck every day at lunch. Actually everything after "I work in a jewelry vault" is a lie.