Deflecto Duct Plastic Increaser and Reducer, 4" to a 3", (IRB43) Connects a 3 in. duct to a 4 in. duct Ideal for installing a bathroom fan Plastic construction for durability White for appeal Duct fits over reducer/increaser easily 4 inch To 3 inch Increaser/reducer Allows Use Of 4 inch Exhaust Hood With 3 inch Duct Plastic Material allows for light weight design