It's not too late to plan a fun 4th in the city if you haven't yet decided on what to do.
Of course the big event is Macy's Fireworks at 9 pm on the 4th. Make sure you have a good rooftop to view them from.
At noon on the 4th, if you happen to be at Coney Island, Nathan's Hot Dog Eating Contest is always a great source of disgusting viewing fun. If you act now, you can try and win a seat as a judge at the contest.
Or, you can just hang out in Central Park with some good friends, a picnic basket, and a cooler full of your favorite drinks. That's my suggestion. Relax and enjoy.