First, the days surrounding the 4th and the 4th are some of the busiest of the year. If you are visiting one of the parks during this time, you need to arrive early and expect to stay for the whole day. Don't plan to park hop and don't plan to go back to your resort for a dip in the pool!
Historically, on July 4th, the parks will reach capacity and the parks will not allow additional guests to enter, even if you have dining reservations inside one of the closed parks.
If you are staying on Disney property, you are guaranteed entry to a theme park, but it may not be the one you want to visit. To make sure you get into the park of your choice, arrive early and stay all day!
Magic Kingdom, Epcot and Hollywood Studios each have special 4th of July events scheduled on July 4th. I could not find anything special happening at Downtown Disney or Animal Kingdom for this holiday.
If you choose to visit the Magic Kingdom, in the past they show a special "Celebrate America" fireworks show followed with the nighttime parade. While in the Magic Kingdom, don't forget to celebrate by visiting the Hall of Presidents, eating at Liberty Tree Tavern and learning about our countries past in Liberty Square!
If you choose to visit Epcot on the 4th, make plans to head to the America Pavilion! There you will be able to see the Voices of Liberty perform on the America Gardens Theatre stage including the Spirit of America Fife and Drum Corps and a visit from Disney characters! Illuminations around the World Showcase Lagoon will feature a patriotic finale.
If you are at Hollywood Studios for July 4th, you can see Fantasmic followed by a special fireworks show set to a rock n roll soundtrack.