On some days, I would tell you that taking a car from the Pop Century to the Magic Kingdom might be a good plan, but most definitely NOT on the 4th of July. Independence Day is always among the busiest days of the year at WDW. There is a very distinct possibility that the parking lots could get so full that they would close to new entries. Moms Panelist Bret describes this in detail
here.
In fact, many of the tried and true touring rules go out the window during major holidays. Normally I recommend touring the park in the morning, taking a mid-day break back at the hotel and then returning for evening events like dinner and fireworks. But on a few days of the year, notably Christmas, New Years Day, and July 4th, your best bet is to get to the Magic Kingdom early and stay there. You can take some down time while you're in the park. At times when I knew I would need a break, but also didn't want to leave the park, I have actually brought a newspaper to the park and gotten an ice cream and sat in a shady corner to read for a while.
Just try to keep your cool as much as possible, relax and enjoy the fabulous fireworks.