Yes they can begin to sell after approval following a phase III. The Phase IV is when the FDA asks the company to continue to collect data often on potential side effects after approval. They also look for more information on optimal dosing or other possible indications for use. Because so many more people are expected to use the drug after approval they will be able to get better data. Also the restrictions on who can participate in the clinical trials will be lifted so they will have a wider variety of patients
Shiller is an economics professor at Yale who just won the economics prize in honor or Alfred Nobel this last year. He is credited for his analysis of housing and forewarned the housing bubble we just went through --if you buy property his Shiller Index of property value is one to watch.