Hi, Tara! Our family has been to the Magic Kingdom on the July 4th holiday, and although it's one of the best fireworks displays you'll ever see in your life, the crowd is unbelievable as well! So, make sure you pack an extra measure of patience!
All that said, capacity crowds are extremely rare, and there are certain policies set in place at various levels to deal with the crowds. Yet, usually Disney resort guests are one of the last groups denied entrance when capacity crowds are inevitable. Moms Panelists
Erin and
Bret have explained the park capacity closing phases in past questions. (Simply click on their names to see their explanations.)
Based on my experiences, check with your resort's concierge about crowd levels and any word s/he may have heard about potential capacity closings before returning to MK. However, since the calendar shows MK Extra Magic Hours from 11 p.m. until 2 a.m., you will most likely be able to come right in and enjoy many attractions after the fireworks. Have a wonderful day and a magical vacation!