It's rare that a holiday will outdo the crowds and congestion of Christmas week into New Year's.
July fourth is very busy, however and you will need to plan for your visit if you are to be happy with the experience! Choosing a park is half the battle. MK and EPCOT seem to get really hammered for this holiday.
If the day is nice, the water parks usually close due to capacity. I'd not want to be at one on this holiday.
Many resorts have little individual celebrations for the fourth. You may want to check into that and consider staying at your resort and enjoying the pool. If not, selecting your park of choice, wisely, being prepared to park hop if needed, and being ready to deal with capacity crowds will go a long way toward your having a good day!
Have fun.