Summer in general is hot and crowded at WDW...and yet we go year after year during the summer!
The 4th of July day is a madhouse. MK and HS will shut the gates about 10:00 in the morning as they'll be at capacity. It is all the bad crowds of Christmas or Easter week with the heat on top of it.
We went once on 4th of July. I think you get the picture of whether we'll do that again.
The other days around the 4th we found to be pretty similar to other summer days that we've encountered in our stays in June and the rest of July.