Arrivng at Bejing Airport at 4:30am. Leaving again at 8:00pm. Do I have time to go to Simitai?

Christine, it is totally enough to go to Simatai, but there is no direct bus to the Wall from the airport. You are advised to charter a car for the round-trip. You can find the car rental counter at the central area on 1F of the arrival hall of T2, or the car rental experience area on 1F of T3.