How to select specific audio inputs in OBS with MOTU M4

I am trying to record a video with OBS using 3 channels of audio into my MOTU M4.

For some reason, OBS only lets me select the interface I want to use for audio input, but not which inputs on that interface. In the past when trying to use my M4 with OBS, I was only ever able to use inputs 1 and 2, inputs 3 and 4 were always disregarded by the app (frustratingly, Audacity is also like this). This has been a problem for me in the past as I have wanted to use inputs 3 and 4 with OBS, but was never able to, always had to unplug things to work around that limitation.

Knowing this, today I expected to record 2 channels of audio into OBS, and my third mic into Ableton Live, which has no problem selecting additional inputs. I would then later add the 3rd audio track in my video editor.

But today when I open OBS, it is now suddenly taking audio from all 4 inputs on the M4, merging both sets to a 2 channel stereo output. I cannot get OBS to disregard my inputs 3 and 4.

How do I choose which specific inputs OBS is taking audio from? What is my best way of recording into OBS with three mono audio sources? I really need OBS to only record ins 1 and 2, is there a way to manually select these and not just record whatever channels OBS feels like recognizing on a particular day? I am not able to make much sense of the Audio pane under Preferences, all of my Global Audio Devices are currently disabled, if I go into the M4's properties under Sources I can only choose between M4 and "default" with no change in operation between them. I'm not really sure how that is supposed to work, or what the purpose is for these cryptic, minimalist settings that don't appear to do anything.

And why did OBS suddenly start recognizing inputs 3 and 4 on my interface even though I haven't updated or anything?

Do I need to consider recording my 3rd channel of audio into a 2nd computer/interface?

Thanks for any info or direction.