HiDPI/scaling on external monitors...

http://www.macrumors.com/2016/09/23/loss-of-resolution-scaling-macos-sierra-update/

So among the displays I'm looking at, some are 4k displays. And I'm wondering, on a 4k display, what does MacOS actually do? Do I get the option of running it as a real 3840x2160 display? Do I get the option of running it as a 1920x1080 with sharper text? Both? Neither? I can't find anything completely concrete, and don't entirely want to spend $1k+ on a fancy monitor without knowing.

(I'm looking at a Dell UP3216Q, but I think it's Too Large, but it got me thinking.)

 

But there's nothing that lets me change font sizes, so... It looks like my options on a 4k monitor would be 1920x1080 or 3840x2160, and only the former would be readable, and it would give me a significant loss in screen real estate. And the 5k monitors would be 2560x1440, in effect, but they'd also be smaller than my current monitor, and words are already pretty small. Frustrating.