The 4" 170cfm is a little noisy, but the majority of the noise is due to the small 4" ducting and alot of air trying to move down a small hole.
If you get the 4'', get a 4'' to 6'' reducer and run 6 inch ducting. Still get the same air flow, just quiets down the noise created by moving air. My fan is in the attic so I don't even hear it, and after I went to 6'' ducting, it's really quiet.
And I concure, that 424cfm is a beast. Sounds like a fighter pilot taxing down the runway, lol.
In all reality though, it isn't the fan that is noisy...it's just the movement of air.