What venting materials are safe to use with this dryer?

Only rigid metal or semi-rigid metal 4-inch diameter ductwork should be used for exhausting the dryer. Do not use plastic or flexible foil venting materials, as they are known to collapse, be easily crushed, and trap lint, which obstructs airflow and increases the risk of fire. The total length of flexible metal duct must not exceed 8 feet. (Pages 6, 15)