Read the comments, pretty much most people agree that they can tell the difference between normal HD and 4k displays (and content) at and normal viewing distances.
That's inconclusive. For example, most people can tell the difference between the SACD and CD recording of a song. However if you take the SACD and convert it to 16 bits, 44KHz nobody does better than chance at telling the difference; the SACD and CDs were mastered differently, as is UHD and HD content.
Of course with HD vs UHD, we are usually talking about sampling rate, not dynamic range (HDR is a thing though). For downsampling, proper anti-aliasing is needed. One good way to check would be to take the UHD content, blur it and then present it on the same 4k display without changing the resolution. Then do the A/B testing and see what the results are.
If you just downsize to HD, then you will introduce aliasing artifacts, and then more artifacts will be introduced when the TV upscales to 4k again, and it's pretty well known that people can see aliasing artifacts at HD resolution.