What type of venting material is required for the Kenmore 796.79272010 dryer?

Only rigid metal or semi-rigid metal 4-inch diameter ductwork should be used for venting this dryer. Flexible plastic venting materials must never be used, as they are known to collapse, trap lint, and obstruct airflow, increasing the risk of fire. The total length of flexible metal duct must not exceed 8 feet (2.4m). (Page 6, 15)