Stop using 4" anything, it's too small and causes a restriction resulting in hotter temps.
Check out
https://bitcointalk.org/index.php?topic=2796117.0 keep in mind I'm not trying to shill the adapter my friend made, heck you can just slide the 6" duct over the miner fans if you want. But the main take away is for you to get rid of the 4" stuff and personally, I'd go not smaller than 6". I've got my lava spewing M3s venting out via 6" duct and so far so good.
If you can not direct vent the fans into some form of hot aisle, I would highly suggest you follow the above advise and use 6". You could alway make adapters for the fans out of stryro or double thickness drywall.