While we were in the world this past 4th of July, we were not crazy enough to brave the crowds at MK. Instead, we went to MGM and then spent the afternoon at the pool at our resort (tower room facing MK in CR). I believe that MK closed for capacity at about 10:00 am. I would plan on arriving early and not leaving again until well after the fireworks. Oh, and btw, it is probably going to be amazingly hot.
So, in short, tens of thousands of your closest friends packed into one sweaty, cranky, tired day. If you plan well, you may be able to pull off some stuff. Definately make some ADR's so that you will at least have a nice, cool place to eat.
HTH and have fun!