Contrast/clarity of the TV and motion artifacting are irrelevant. There are good quality and poor quality HDTVs, and the same will true for 4K TVs (there is already a 50" 4K TV that costs less than $1000 from Seiki, which I assume is not of the highest quality) 4K will use AVC encoding which reduces the required bit rate but you'll still see cable/satellite providers overcompressing signals to fit more in the available bandwidth, or at least they'd have to if 4K was used on more than a handful of channels. These are the same problems that HD faces, but it still was a big improvement over SD. If your argument is that 4K is better if you have a Kuro-quality TV and Blu Ray source material, well, even if so most people will have a LCD which inherently has poor contrast, buying one or two steps up from the cheapest model at the size they want, and be watching cable or satellite signals compressed 4-10x more than Blu Rays are.
But the main reason why the quality of the TV and source material isn't relevant is because they didn't do actual testing to make the charts, they based it on the visual capability of the human eye. The eye has only so much angular resolution possible due to the density of the cones on the retina, and that is only available on a fairly narrow area at the center of the eye - the rods that give you your peripheral vision have terrible angular resolution, and can't perceive color, but are very good at sensing motion. Your eye is constantly makes small involuntary movements to increase the range of vision by moving the cones around, if it didn't we'd all have tunnel vision, able to see clearly only a few degrees to either side of our nose (actually, due to the way the brain works, if the involuntary eye movements are stopped, you are completely blind)
The lens in the human eye can only focus so well. The charts assume 20/20 vision, but those who have had LASIK with really good outcomes would be better than the charts indicate. If you've had wavefront LASIK and ended up with 20/10 vision, you can cut the distance on the charts in half, and would probably see some benefit from 4K at normal viewing distances. The same would be true if you are one of those who likes to sit much closer than the 'recommended' viewing distance for a screen of a given size. Most people find it tiring to sit that close for very long, because you have to move your eyes too much to take in the entire screen. But the people with vision in the 20/10 range are a tiny fraction of the population, and while I have no idea how many people sit "too close", based on where friends have their couches for their TVs I think most people naturally sit at the recommended distance, just like most people going to a movie theatre naturally avoid the first row.
To the point, have you actually SEEN a fair test of a 4K TV? I had the opportunity to see one recently. They had a demo playing on it from a connected computer, showing all sorts of flowery meadows and a few movie clips meant to show off how wonderful 4K was. The most interesting part to me was where it showing a split screen view of a 4K movie, with 4K on the left side and a 1080p on the right, downsampled with four pixels used per pixel to simulate 1080p. Of course the difference was quite visible from up close, but when I moved back to a distance I'd find comfortable to watch, I couldn't see any difference. That's not surprising because even with contacts I have slightly less than 20/20 vision due to a mild uncorrected astigmatism. I moved in a little closer, a little closer to where I could start seeing the difference, and didn't find a 'small' difference all that dramatic. Even from right up close where the difference is easy to see, it was nothing like the dramatic night and day difference going from SD to HD.
In the past when I've seen 4K it has been some high end high dollar 4K screen showing 4K source material, next to a lower quality 1080p TV showing something totally different (or the same movie a bit out of sync, clearly demonstrating that it wasn't using the same source material) That's rigged, IMHO, and not very useful for telling the real difference between them that one's own eyes can perceive.