Hi David,
I have recorded and Open Mic’d with both the UMC404HD and the Scarlett 4i4 3rd Gen (my current device). I found both good to use and felt the 404 was very flexible with it’s input/output options although my subjective feel was the 4i4 sounded better plus its software provides some good functionality. In saying that I think both are great AIs so can’t go wrong really.
From memory, I think I only went straight into OBS with the 404 without using ASIO at the time so all channels were combined. I was feeding in my guitar, vocals and a backing track (with all effects on my guitar and vocals prior to the 404) so really I just wanted everything to be simple and synced when captured by OBS.
However, now I’m using my 4i4 via Reaper to OBS as per Toby’s instructions here.
So I haven’t used either with all 4 channels accessible separately in OBS in a production situation. However, as a quick test, I have checked it now with the 404 and appears to work when using ASIO4ALL and the OBS-ASIO plugin, if this is what you are looking to confirm? 
Hope I have understood correctly! If not, apologies! 
As I have both a 404 and 4i4 accessible to test, please let me know if there is anything else you would like me to look in to 
