We were at WDW July 4th week in 2013. We went back to the room midday every day to swim and take a break from the heat/crowds. We normally aren't "afternoon break" people, but we had to in the summer.
We got A LOT of ran during the trip. It was more than the typical summer showers. That helped with the heat/crowds. So I'd definitely be prepared for rain. You will, at the very least, get the daily summer shower.
For the 4th, we went swimming in the morning. Early afternoon we went to AK and stayed until it closed, then jumped on a bus to Epcot to catch the fireworks. We were able to grab a spot in the grassy area across from the Imagination pavilion. It was crowded, but not too bad, and not as bad as being around the lagoon.
I'd definitely consider doing the water parks. My oldest son, 10yo at the time, loved them. It was the first time we went to them.
One evening, we left our younger kids with grandma, and headed back out to MK around 10pm with our oldest DS for the super late EMH. We had a great time!