I have about 4 hrs in Chongqing before boarding the Yangtze river cruise.

During these four hours, you can go to CQ China Three Gorges Museum. It's about 2.5 miles away from Chaotianmen Port. So after touring around, you can take a taxi to the port and start your river cruise easily. The cab fare is CNY 15 to 20.
In addition, you can also go to Jiefangbei Area. It's one of the most properous areas in this city. The cab fare to the port is around CNY 20.