That is again about content, right?
But if you sit far enough from your 4K screen, so that 2 pixels fit within your best visual acuity (ie 1 arc degree on average, every individual can do better or worse than that), then you won’t be able to tell if your screen has 1 or 2 pixels in that spot. So, you are effectively seeing a half of resolution, that is 1920 in horizontal direction. The farther you move, the bigger the screen area gets where your eyes won’t be able to tell details apart.
So, you’d want to move close enough to see the difference between those 2 neighboring pixels.
PS I agree, to even be able to play 4K content, you need a 4K screen to begin with.
While pixels is a talking point the tangible benefit of 4K is that more of the screen is in focus e.g if you pan around the image things beyond the foreground like signs, shop fronts etc are more legible. This is true whether I'm sitting at the "optimal" distance or further away, and is the end goal rather than being able to perceive twice or four times the pixels.