Hello, Jennie!
You are in for a very special treat for the Fourth of July at Walt Disney World!
First of all,
Disney's Hollywood Studios does have a Special July 4th Fireworks Presentation that my son and I were fortunate to catch not too long ago. Ask a Cast Member at HS if there will be a practice run on the 3rd, in case you happen to be there then.
The reason I suggested visiting HS on the 3rd is because you really need to be at Magic Kingdom Park for the Fourth of July. After one viewing of "Celebrate America: A 4th of July Concert in the Sky," I actually booked our next trip over the same holiday, just so that we could get a repeat of that show. (By the way, we were able to watch a practice run on July 3rd of this display, as well!)
Keep visiting Magic Kingdom's calendar page for this date, so that you can find more information as it becomes available. Just be sure to be inside this park as early as possible on the Fourth, because it is very popular, and may close (due to reaching capacity) earlier than you may think. (And yes, the crowds will be big, but that show is worth it!)
Finally, fan site allears.net reports that many Guests also enjoy an enhanced IllumiNations: Reflections of Earth on the Fourth of July. While I've not yet attended this one, there are often additional characters available for meet-and-greets at Epcot as well.
The Walt Disney World resorts, too, have had patriotic-themed events poolside, including red, white and blue beads, noisemakers and trivia contests. We even met Patriotic Goofy at Old Key West several years ago on the Fourth of July, another celebratory surprise in a trip full of many!
Please send additional questions as you think of them! I'd love to help!