So, I went to Disneyland on July4th this year (and DCA on July 3rd) and July 4th was WAY less busy than I expected. I am an AP and I go to Disneyland at least once a month, I went during Christmas times, holidays, ect and July 4th was just an average busy summer day, nothing bad. The only crazy part was the fireworks; people started to reserve their spot in front of the castle at 4pm (after the last parade). I would pick July 4th for another visit anytime (unless it was Saturday).
Also, as a note: this year, 4th of July fireworks were on the 3rd and the 4th.