Vent Systems 6" Inch to 4" Inch Galvanized Steel Duct Reducer with Rubber Gasket - Vent Adapter - Fitting Pipe - Connector, Increaser, Reducer, Cone - HVAC, Ventilation Systems

Snug fit, works prefectly.

Revised:TL;DR: Excellent reducer, BUT the 8" side was small -- didn't measure, but likely 7-7/8 (ignoring that, it was chewed up, too, and not because of shipping). I had to crimp heavily going into it. But the 4" side was proper -- about 3-7/8", so it'll slide right into 4" flex. No leaks, heavy duty.Review: I got a used, beat up one in a perfectly unharmed box. but this is the one you want, so I dont want to give it low marks and turn you away. My dad was an AC guy. He got screwed up stuff from the supply house all the time, but it was still mostly serviceable if you had a hand seamer, snips, and a crimper, and we had work to do. This was serviceable, too. It is a much nicer reducer than I'd find elsewhere. Hopefully the seller will look over quality control, and hopefully *you* will be able to sift through reducers to find this one, or one of matching material and design quality. It won't leak from the seams, is heavy gauge, and is unobstructed by seams/design -- that's what you want. You'll get strong, quiet air flow through this reducer, and you won't have to stack reducers like a sheet metal christmas tree.

Be careful - the main page states 6' X 4" duct reducer. But it's accurately metric sizes ( 150mm to 100mm). I purchased it to fit into a roof vent. Luck for me I needed 4 roof vents and each stated 6" diameter. The last one turned out to be about 6 1/4" and it fit. It states "Easy to install - fit perfectly with any other pipe, vent, and connector. Push-fit duct and fittings together." Not true because of the metric size. Just make sure you know the size of what it's connecting to. Fits tight but still should have a screw or two to hold it in place. That silver duct tape would also work. Hope this info helps.

Used to accommodate a 10 inch booster fan in an 8 inch round duct.High Quality, precisely fabricated, the galvanized steel is gaged sufficiently to be very rigid.

Good quality, great price.

Works as advertised

Perfect just what I needed. 4" to 8" just a little tweaking and modifications and it connected right up from 2 different manufacturing designs. I really appreciate it's heavier gage and the rubber grommets type seals. This is my first mod build. I'm sure the 8 inch expansion will give that free flowing boost, eliminating conjunction backups. DIY one timeIdea. Will up date if needed. Definitely a well made product. Thank kindly, God Bless and Much Aloha's. GT

Liked the ruber gasket for a secure fit.Great products