Broan Model RVK1A Roof Ducting Kit for 4" Duct.  Contains the installation supplies you need to install your fan.

Features:
• Includes 8 ft. of 4", flexible, 2-ply metallic laminate ducting, 4" diameter metal duct connector, 2 duct clamps, 4" to 3" reducer and black steel roof cap with built-in back draft damper and bird screen
• Duct conforms to UL181 Class 1 air connector requirements
• Not recommended for dryer venting