What type of venting material is required for this dryer?

Only rigid metal or semi-rigid metal 4-inch diameter ductwork should be used — do not use plastic or flexible foil venting, as these materials are known to collapse, trap lint, and increase the risk of fire. The total length of flexible metal duct must not exceed 8 feet (2.4 m). All duct joints should be secured with duct tape, not sheet metal screws. (Pages 4, 16)