Can i please give you some advice?
Spend most of the day doing things outside the park. Remember that locals from hours around all go to WDW on July 4. It's a "mad house".
If you dont get to MK early , you get locked out basically (not all of the time but in many cases) due to capacity.
However, here's the thing, the best fireworks show (in my opinion) is at Epcot.
I've never run into a situation on July 4 with Epcot closing due to capacity. So, spend the day outside of the park and then go in around 7:00. Trust me, to stay in the MK all day just to "soak" up the atmosphere will ruin your day.
IT'S MISERABLE!
The attraction wait times are 2 hours long (at both Mk and Epcot, so dont go to do attractions per se).
Basically in my opinion, if you go on July 4, you are going to see the fireworks. There's just not much of another reason to go on that day.
Epcot's fireworks are longer and MUCH better than MK's.
Also, just a hint: MK does it's July 4 show on July 3 as well. It's the exact same show. You can go on that evening and see the MK show with less crowd (though that's all relative, as it's still HEAVY).
Then on July 4, you can to go Epcot and watch that show at 10:00. The MGM show is at 9:00 and you can see it easily from Epcot, so you get to see all three shows on your July 4 vacation.
I think if you go to MK on July 4, you'll be doing the whole "they told me so" routine.