Why is Audacity not taking input from this device?

You’re welcome. It’s good to know that the IO4 does work on Linux.
We don’t often get to hear which multi-channel devices work with Audacity, particularly not on Linux. Perhaps if you have time you could post a short review of the IO4 in this topic: https://forum.audacityteam.org/t/multi-channel-recording-in-audacity/15644/1