Broan-NuTone NuTone WVK2A Flexible Wall Ducting Kit for Ventilation Fans, 4-Inch

It was perfect fit

I used this to replace the vent that was first installed in the house. The original vent had plastic louvered slats that had warped over time. Whenever there was a hard rain, the louvers let water in, and it was leaking inside the house. I like the sturdy single flap that won't deform to let weather in. So far it's working great.

Kit has everything you need to install a vent to a fan and all the pieces feel to be very good quality. *Note - don't be a moron like me and buy 4" hole saw to install this. You'll need 4.5" to allow clearance for the tube and white vent piece. Once I cut the correct sized hole, everything went together quickly. Also, maybe purchase some tin duct tape if you want an airtight seal.

Easy to install. Works great

Not the highest quality, but it does it's job. Other than having to buy a hole saw to go through the outside wall, installation was quick and easy. Will rarely, if ever, be used so it should work.