Once that's all good and you're getting the appropriate resolutions being read in the 'grabber' app, go to OBS and select your first input.

Finally, as shown in the attached image of the Camlink Pro Device Properties in my first post, its video is "3840x2160p" with a refresh rate of "30fps" This is why I believe it is something in OBS. Read more about the condition New: A brand-new, unused, unopened and undamaged item in original retail packaging (where packaging is applicable).

In OBS even if I have canvas and output set to 3840x2160, and add the source capture video configuration at 3840x2160, OBS seems to see it as a 1920x1080 device. If the item comes direct from a manufacturer, it may be delivered in non-retail packaging, such as a plain or unprinted box or plastic bag. Run original software for this grabber and check value on sender and receiver, now run OBS and check.

However, with all this information, you should now be able to run 4 separate 4k30fps inputs inside OBS. So my theory is that even when you set OBS FPS to a low value (24 or 30), it's still trying to connect to the source video capture device at a HIGHER FPS, and that is kicking the Elgato Cam Link Pro into a lower resolution capture.

Then when I rescale it all back down to 1080p, I am right back where I am now, with my 4k source scaled to take up the entire base resolution. I really think it's worth taking a look at my log above showing that OBS is negotiating with the device for 1920x1080 at 50FPS.