Product review (3 stars): ... and accessory manufacturers sit down every day and area like "yes, make that OD exactly 4
For some reason all 4" ducting manufacturers and accessory manufacturers sit down every day and area like "yes, make that OD exactly 4 ... this is a good idea"

It's not!

Jesus I think I sliced my fingers open a few times trying to get my ducting on this thing. VERY SHARP, EXACTLY 4" ... STRUGGLE FACTORY.

Seal with some aluminum tape as you will definitely be getting some holes in your ducting just trying to slip it on