Phase IV are post-marketing studies, frequently on groups that were not included in earlier studies, pregnant women, children, people with other conditions that excluded them from the phase III studies.
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