We were there two years ago and avoided parks entirely on the 4th. A nice CM told me about being trapped in "a sea of bodies" on a previous 4th when her shift at the MK ended, and that didn't sound like fun to me. Instead, we swam and relaxed all day. There are poolside activities at some of the resorts. That said, the MK fireworks are not to be missed. We had dinner reservations at the Polynesian and watched from there. It's hard to describe how wonderful the fireworks were -- you didn't know where to look because so many were exploding.
I have read here that Polynesian guests have been upset about the onslaught of visitors who want to watch the show from the pool/beach. If it were me, I would make a late dinner reservation at 'Ohana or somewhere at the Grand Floridian and watch from either of these resorts. Especially at the Polynesian, I doubt they will allow visitors to park and visit that evening. But if you have a prioroty seating you'll be allowed in. The view of the fireworks from 'Ohana is great.
PS: We love Coronado. The grounds are lovely, especially at night. I have been told there is even a little alligator in the lake, but you have to look for him early in the morning! The pool is one of the best on property. Enjoy!