That’s sort I’d the point: that you can’t see the pixels … Text looks amazing because you don’t have to anti alias the jaggies away now that the pixels are so tiny. I much prefer text on a very high dpi monitor because everything just looks so crisp. Like a really good printed page, or something else physical. Going back to a low dpi monitor is real hard for me now—pixelated text is really hard to look at.

Well, you wouldn't run the computer at a 1:1 resolution, it would be a logical lower resolution displayed at a higher resolution. For a 4K display, everything would show at the size it would be on a 2K display — but actually rendering at 4K, everything is much crisper rather than just blown-up pixels.

Apple calls it Retina Display, Windows calls it display scale. Apple made it mainstream in 2010 (iPhone) and 2012 (iPad, Macs) whilst Microsoft seems to have made it much more mainstream since Windows 8 and the introduction of its Surface computers.

Depends how close you sit to it (touchscreen models promote closer use) and how good your eyes are :)

I can definitely see the difference between 4K and Full HD on a 15".