Best hardware for 4k video SDI input / run OpenCV for overlay / 4k video SDI output with reduced delay?

Because I want to end-up with some idiot-proof black box, with just a power source, an off-on switch, and 2 SDI ( input and output)

That is great I did not know it was that easy.

Thx mate ! You are very helpful indeed. I guess I will narrow my quest to Nano, and maybe compare it to the Open Ai kit which is said to have more computational abilities, but no Cuda ( but I may be wrong ). I think the Raspberry 4 has a VideoCore IV GPU

Apparently OpenCV works with OpenCl ? That means the Raspberry 4 should also be quite capable ?