This year is the first that I can remember a 4th of July coinciding with Shabbat (I'm sure it's happened...I just can't remember!). Of course, that poses a problem for Shabbat-keeping families. What's more important - G-d's holy Sabbath or America's birthday? I read a great post about this at First Fruits of Zion.
Have a happy and safe Fourth of July and Shabbat Shalom!