Welcome to planDisney, Jill.
How wonderful that you'll be celebrating Independence Day at Magic Kingdom Park (the irony of an Englishman uttering those words is not lost on me). You're going to love
Disney's Celebrate America! A Fourth of July Concert in the Sky. At some point, make sure you head over to Liberty Square and visit the
Hall of Presidents. I think that's what Walt would do!
If you have the Park Hopper Option, then you can absolutely hop to another park after 2:00 PM. I'd suggest going to EPCOT and watching
Voices of Liberty. It's also the final day of the
EPCOT International Flower & Garden Festival. You can grab a bite to eat and then head back to Magic Kingdom Park for the fireworks.
Even when theme parks are at capacity and therefore no longer taking theme park reservations, Guests can usually still park hop if they have the Park Hopper Option. I believe there has only been one occasion when a theme park was so full that Guests couldn't park hop to it. So chances are that you should be ok to return. However, it's not guaranteed. You'll be taking a risk, even if it's a small one. You can check the My Disney Experience Mobile App and it will let you know if Magical Kingdom Park is no longer accepting Guests who are park hopping. If that were to happen, you'll still be able to watch the Fourth of July fireworks at EPCOT.
I hope that's helpful, Jill. Have a magical Fourth of July!
Mike