I don't know why anyone my age needs a 4K monitor. What's the point when you're gonna set everything to scale double to read it....lol.
That's exactly the point.... I work all day on a 27" 5K display. All 3 MacBooks in the house also have HiDPI Retina displays. HiDPI on the Mac is a bit different, but essentially it defaults to the equivalent of 200% scaling. Thus, everything is the correct "size", but everything looks very crisp/sharp.
It's extremely noticeable when you then flip to a non-HiDPI display. When I was on vacation two weeks ago, I had to "fix" my mom's computer as well as setup email and a few things on my niece's computer. While the fonts and controls and such were all roughly the same size, the text looked horrible compared to what I'm used to. The pixels are definitely noticeable, the cleartype blending is noticeable, etc... As a developer that needs to stare at text all day, having the sharpest and clearest text is important to me.
That said, HiDPI on a Mac is *way* easier than on Windows (or Linux, Linux is even worse). It's pretty much automatic.