You would have the time to visit the Sultanahmet area (Blue Mosque),Hagia Sophia. If you decide to do this, and if you need a visa to enter Turkey, I would apply online to save a bit of time in the Airport at immigration. Taxis are available for about $20 US each way from IST to this area. Public transportation is much cheaper (about $3 US). You need to be back at the Airport by 18:00 for security check and airline check-in; if you have already your connecting boarding pass it will be quicker. Make sure to have any luggage tagged to your final destination to avoid waiting at customs area. Good luck!