When you need to change from a 6-inch duct size to a 4-inch duct size, use Active Air's ACR0604 duct reducer. It's a perfect fitting for maintaining maximum airflow through your system that requires a specific size duct. Simply slide the reducer into the item you are attaching to and fasten with sheet metal screws or duct tape (not included). These galvanized duct reducers can be used with fans, reflector hoods, ducting, and ventilation systems.