we have 9 hour layover in hong kong airport and we like to do sightseeing in the city for 4-5 hours.

No, US citizens don't need a visa to get out of the airport. As you only have 4-5 hours, I suggest you take shuttle bus line S1 to Tung Chung station. Then walk to the cable car station to catch the cable car to Ngong Ping 360 (Po Lin Monastery and Giant Buddha) for a visit. The crystal cable cars are worth a try and you are recommended to buy the round trip ticket to save money and time.