Crowds on the 4th of July are rivaled only by New Year's Eve (they are even lighter than July 4th some years).
If you want a good spot for the fireworks on Main Street, you want the front of the hub around noon (possibly earlier). There are other good places for the 4th of July fireworks though. Some of the good places are: Rivers of America, it's a small world and the outside of the second story of innoventions. The first two locations you will need to show up 3-4 hours early, the last location you can get away with 30-45 minutes early.
Again, this only applies on the 4th. There is a slightly shorter version that runs July 1-3. Also, there is a new twist that might change things this year and that is the special 4th of July world of color show. But I doubt it will affect much on the actual day of the 4th. Both parks are packed. I would also not expect to get into DL after about noon, unless you are staying onsite or have already been in that day.