Sorry I missed this. Since my last post I have an AJA Kona HDMI card which works fine. The unreal plugin for it currently has some bugs with more than one 4k stream (it should do 2) but they are looking at that problem.
The problem here is simply drivers. It appears that anything that shows up on your PC as a webcam (ie: in the “camera” app) should work with Unreal. If your card does NOT show up like a webcam (AJA and BlackMagic cards) then you need an unreal plugin that supports it. If you have an AJA or Blackmagic card, check the compatibility list for the plugin on the unreal marketplace. If your card is not listed it might still work but probably not.
One other trick which is ugly, if there is an OBS plugin for your card I think OBS can make it’s output appear like a webcam which Unreal should be able to see.
I haven’t been able to find anything else as cheap as the Elgato Camlink 4k. I think that any USB capture device that says it lets you use your DSLR with “video conference” applications or “as a webcam” will probably work. There are a fair number of these out there but most are in the $300 price range. The Elgato was $99 but since Coronavirus hit the price has jumped to $130-150 and they are pretty much out of stock everywhere.
Also fairly recently Canon and Fuji have released software for their DSLRs that let you use them as webcams.
Greg