REMILICIOUS, IF you have a valid US visa - sure, go out do some sightseeing. Heck, you can even intentionally miss your flight.
POSHPRINCESS - if you land in NYC to connect to a domestic flight - you need to be at the airport 1hr before the flight, or checked in at the counter a minimum of 30min. before the flight. (if international - 2 hrs min. before the flight.
Working backwards, that's 4 hrs left. Now, offloading from an international flight -- it takes an hour more or less to clear Immigration & Customs. Longer if there are 3 or more international flights landing. so, you have 3 hrs left. Give it an hour 1way fom the airport to the city, then back, given traffic. So - you have an hour to kill -- but you will most probably be tense and harassed, and not enjoy your quickie "tour".
Why don't you break your trip - spend a day or more in NYC before resuming your onward itinerary??