Hot and crowded are understatements.
I'll say...
I don't know how many times you and yours have been to the parks, but if you're going to the MK with the desire of, "we're going to ride all the rides and see everything possible..." Well, frankly, you need to put that idea out of your head as quickly as possible.
This is not to say that the 4th is not an exciting day to visit Disney, (just the opposite, actually) but if your focus is the Magic Kingdom then you need to accept a few things:
1) The park will most likely reach capacity by noon, which means NO one (even those staying at hotels on property) will be allowed entrance.
2) No matter HOW much of a Disney fan you may be, that "magic" can quickly wilt in the face of oppressive heat, lines for restrooms and counter service restaurants and a guaranteed lot of "guests" losing their cool during the day.
3) Disney knows all too well the "chaos" that happens during this holiday, and has plenty of non-ride related shows and activities to help alleviate this problem. This is a day to people watch and enjoy the holiday, NOT a day to try and see everything, ride the three "mountains" and the HM and Peter Pan's Flight. The lines are endless, and the mood can be less than magical.
On the 4th itself, I'd *strongly* recommend touring Epcot. It handles large crowds the best, and it's "open air" design helps mightily with the heat. Plus, "Illuminations" has a really nice little *zing* on this night. (Not to mention there is beer and tequila just steps away...) (Did I say that out loud ?

)
Also, remember you kind of have to play it by ear. On the rare times we visit the park during the fourth, we stay at the CM, knowing full well that once we exit the park we MAY not be allowed back in. But, the view of the fireworks from the balconies on the park facing side of the hotel can be just as magical. And being able to WALK back to your resort is WELL worth the extra $$$
But all is not so dreary as I might be describing it.
As Kevin said above, the fireworks on the third are pretty much the same display, and remember, regardless if it's the third or the fourth, one of the best places to view the show is on "the beach" at The Poly. (Shh ! Don't tell anyone !

) Simply hop on the monorail, a bus or boat headed for that destination.
If you insist on "park hopping" that day, I'd recommend the MK EARLY in the morning, then DHS or AK midday (although the AK will be swelteringly hot) then Epcot to finish off your day of exhaustion.
Although, at this point, if I were you, I'd just get to the MK before the rope drop and drink in the magic of where you are and what you are seeing from a nice park bench in the shade.
But, I'm guessing your family may have different ideas.
