I recently bought a Datapath VisionLC HD2 HDMI capture card and I wanted to share my success.
Above you can see it capturing an output from isadora running at 1920*1080 and up-scaling it to 3840*2160 (see below). I have since read it can capture a single HDMI at full 4k - which I will try one day.
The hardware cost £435 which is pretty cool as this can capture DUAL HDMI inputs. So its almost £200 a channel input capture.
Frame delay is minimal but the quality is amazing.
It's also:PCI Express Gen 2 x 4 low profile
And it is indeed low profile! I was very tight on space.
I have done now hardcore testing yet but it works and I will keep you posted on my research.