I'm not sure it works that way. Just because you can't see pixels individually doesn't mean the overall picture quality won't appear better. I'm too lazy to do the math, but I'm guessing on a 25" screen, by his math, I wouldn't be able to see an individual 480p pixel, but I'd still take the Pepsi challenge between that and 720p.
And sure, a high contrast 720p screen might look better than a mediocre 1080p, but that's not a fair comparison. It doesn't mean that given equal quality, higher resolution isn't better.
The real reason not to buy a 4k TV is that content is non-existent and probably will be for a few more years at least.
Old content is also unlikely to end up being re-encoded to a size above 1080p. Old TV-shows are already having problems getting to 720p, and shows like star trek has to re-render things to get up to 1080, and shows like babylon 5 has yet to make it to the 720p mark.
Actually while this may be right for some very old or very cheap content, movies tend to be stored on much higher resolution material - Either way if 4k is not a problem, 8k will be, so it remains an increasingly important concern.
And sure, a high contrast 720p screen might look better than a mediocre 1080p, but that's not a fair comparison. It doesn't mean that given equal quality, higher resolution isn't better.
The real reason not to buy a 4k TV is that content is non-existent and probably will be for a few more years at least.