Okay, 4th of July is on a Friday this year. The week of June 30th - July 4th will be a big vacation week for some families. DL/DCA will be more crowded than what us locals are used to (so many of us will stay away

). Yes, there will be LONG lines for just about everything. When you can, use FastPass. When you can't (either FP isn't offered or they are gone for the day), then try to shoot for hitting the more popular rides @ park opening, right before park closing, or during parades & Fantasmic.
Forewarning: if it is hotter than Hades, then the water rides (Splash Mountain & Grizzly River Run) have been known to have lines pushing the THREE HOUR mark (saw it last summer). So if you want to ride those, definitely grab FastPasses first thing in the morning. Side note: Splash Mountain
sometimes has a single rider line. Ask a CM as soon as you get to the area, no use wasting your time in the stand-by line if you can do single rider in a fraction of the time. The Indiana Jones ride and Soarin' over California also have single rider lines
sometimes. It is always worth it to ask a CM.