Where are you staying? We have gone to WDW for the past several 4th of July's and we stay at Poly. From there, you can watch the entire Wishes show, including the water show -- it's fabulous! However, if you are a non-Poly guest, I believe they are trying to crack down on the crowds there so I'm not sure I'd recommend going there for the show.
In past years, they've shown the 4th show on both July 3 and July 4. We actually go to MK on July 3 and then hit a different park on July 4. Even on July 3, the MK is very crowded. What we do is get there early and go on our FP rides and any other rides we can. Since we stay at Poly, we head back to the resort by 6:00ish for the 4th festivities there and wait for the show. If you're not staying at a resort to view the show, you could watch the show from most areas in the MK and see just fine. The trick will be to find a not-so-crowded spot to watch. We've watched regular Wishes before from the entrance/exit area of the park and it has actually been a beautiful view. Then when the show is over, we can skirt out of the park without dealing with the crowds.
Also, people have said the views from the TTC are good (the boat launch area). There are viewing spots from the CR and that is attached to the MK via a walkway.
As far as a time to get to MK, the earlier the better in my experience. It only seems to get more crowded as the day goes on.
Have fun -- that is one of our favorite weeks to be in WDW!
