I've never been in the parks on the 4th proper, but I have been quite a bit in July. As long as your expectations are in line with reality, you can have a great time.
1) It will be hot. Really, really, hot. And not a dry heat. No, a wet, sticky, "I'm meeeelllting!" heat. Plan accordingly. Cool clothes, lots of water bottles, and take advantage of morning and evening extra magic hours. When I'm there in the heat of summer, I prefer to do the parks under the cover of darkness -- it's less crowded and it's less hot.
2) It will be crowded. This is surmountable. Use touring plans, get there early in the morning, do some things, get fast passes, and then get out when things get too hot and crowded. Come back in the evening. One of the great things about the summer is that there are super-long park hours.
3) It's gonna rain. At about 3pm every day. And not just rain, giant, world-is-ending-style storm. Learn to love it. Bring ponchos, plan to do indoor sorts of activities at that hour. Planning to go hit Test Track at 4pm on a July day is unwise. That's a great time to be in the Land pavillion. This is another reason I always bring a full change of clothes, from shoes to underwear, in case I get soaked in a downpour.
As long as you go in expecting these three things, you should be fine. It seems like the people who have the worst time at WDW in the summer are the ones who are SHOCKED to discover that it's hot in Florida in July, or that the parks are crowded on holidays. You can still have a great time, as long as you plan appropriately.