This was an issue discussed more in the past, when 4k was first coming out. For differences in resolution, seeing a difference depends on basically three things:
1) your eyesight
2) your viewing distance
3) the screen size
All three matter; change any one of them, and you change when the higher resolution matters. Here you can look at an article and a chart and a calculator:
Because different people have different eyesight, they normally choose 20/20 vision for these sorts of charts. (It would be good if someone would make a calculator with eyesight as another variable, but I have not found any that allow for that and only are correct for 20/20 vision.) If your eyesight is better than that, then the distances will be greater when the resolution starts to matter. (That is, if you have better than 20/20 vision, then you will see all of the detail of a particular resolution TV of a particular size at a greater distance than someone with 20/20 vision. This means that, with your better than 20/20 vision, you are going to find TVs at the maximum distance on such charts as being too close such that they do not offer perfect clarity.) If your eyesight is worse, then the distances will be less before the resolution matters. If you watch with glasses or contact lenses, then it is your corrected vision that will matter for this.
Looking at the calculator, if you have 20/20 vision, with a 100 inch screen, you can see all of the detail of 1080p at 13 feet. So with 11.5 feet, you could see a difference with true 4k content, assuming you have 20/20 vision (which we have NO
reason to assume is the case for LeisureDave).
For those who are wanting 8k, one would need to sit closer than 6 feet to a 100 inch screen to see any difference in resolution between 4k and 8k, if one has 20/20 vision.
It is also worth pointing out the fact that there are other differences in TVs aside from resolution, and those other differences may be visible even if the added resolution is not visible. So one may well see a difference between two TVs when the difference one is seeing is something other than a difference in their resolution.
If your vision is exactly 20/20, then those calculators can be used as-is. If your vision is better or worse, then those calculators and charts will be off a bit, with the more your vision differs from 20/20, the more off they will be. In all cases, if you are seeing individual pixels from your viewing position, you may wish to invest in a higher resolution TV (or just sit further back, or get a smaller TV).
Keep in mind, your preferences also enter into this, as some people are more concerned about perfect clarity than others, and some want a larger screen than others for a particular viewing distance.
Also, many people judge this sort of thing in the wrong way, by putting their faces really close to the screen to see if they can see a difference. It does not matter if you can see a difference at a distance closer than you will be actually watching it; what matters is what you can see at the distance you will be viewing the screen. Salesmen, though, may like for people to get up closer than they will actually view the screen, so they can sell people a TV with higher resolution than the person needs.