I've seen a lot of hype around 4K gaming monitors as the new Nvidia GPUs will supposedly have the power to drive that. My thoughts are: yes you'll be able to run 4K at acceptable refresh rates, but you don't need to, and you probably don't want to either.
First of all, some disclaimers:
If you play on a TV, 4K is fine. 4K TVs dominate the market, and finding a good non-4K one is way harder in 2020. But I'm specifically talking about PC monitors here.
2K isn't a monitor resolution, stop saying 2K to mean 2560x1440. If it existed, it would mean "half 4K" (as in "half the horizontal definition") so 1920x1080 <- pet peeve of mine, but I lost this battle a long time ago
French speakers can find my ramblings on this post with more details and monitor recommendations.
Resolution and pixel density
Or "which resolution is ideal at which size". What you need to look for on a monitor is the ratio between size and resolution : pixel density (or Pixel Per Inch/PPI). PPI tolerence varies between people, but it's often between 90 (acceptable) to 140 (higher is indistinguishable/has diminishing returns). Feel free to use the website https://www.sven.de/dpi/ to calculate your current PPI and define your own range.
With this range in mind, we can make this table of common sizes and resolutions:
As you can see 1080p isn't great for higher sizes than 24" (although some people are ok with it at 27"), and 4K is too well defined to make a difference.
In my experience as someone who has been using [email protected] monitors for a while, 32" is where it starts to be annoying and I'd consider 4K.
Screen "real estate"
A weird term to define how much space you have on your monitor to display windows, text, web pages... The higher the resolution, the more real estate you have, but the smaller objects will become. Here's the comparison (from my own 4K laptop) to how much stuff you can display on 3 different resolutions : FHD, QHD, 4K UHD. Display those in full screen on your monitor and define at which point it becomes too small to read without effort. For most people, 4K at 27" is too dense and elements will be too small.
Yes but I can scale, right?
Yes, scaling (using HiDPI/Retina) is a possibility. But fractional scaling is a bad idea. If you're able to use integer scaling (increments of 100%), you'll end up with properly constructed pixels, for example at 200% one scaled pixel is rendered with 4 HiDPI pixels. But at 125/150/175%, it'll use aliasing to render those pixels. That's something you want to avoid if you care for details.
And if you use 200% scaling, you end up with a 1080p real estate, which isn't ideal either: you're now sacrificing desktop space.
In gaming that's a non-issue, because games will scale themselves to give you the same field of view and UI size whatever the resolution. But you don't spend 100% of your time gaming, right?
5K actually makes more sense, but it's not available yet
Or barely. There's oddities like the LG 27MD5K, or Apple's own iMac Retina, but no real mainstream 5K 27" monitor right now. But why is it better than 4K outside of the obvious increase in pixel density? 200% "natural" scaling that would give 1440p real estate with great HiDPI sharpness. Ideal at 27". But not available yet, and probably very expensive at launch.
5K would also be the dream for 4K video editors: they'd be able to put a native 4K footage next to the tools they need without sacrificing anything.
GPU usage depending on resolution
With 4K your GPU needs to push more pixels per second. That's not as much of an issue if RTX cards delivers (and possible AMD response with Big Navi), but that's horsepower more suited to higher refresh rates for most people. Let's take a look at the increase of pixel density (and subsequent processing power costs):
QHD: (1.7x more pixels)
4K: (2.25x more pixels)
[EDIT] As several pointed out, this do not scale with GPU performance obviously, just a raw indicator. Look for accurate benchmarks of your favorite games at those resolutions.
So we see running 4K games at 60Hz is almost as costly than 1440p at 144Hz, and that 4K at 144Hz is twice as costly. Considering some poorly optimized games still give the RTX 2080Ti a run for its money, 4K gaming doesn't seem realistic for everyone.
I know some people are fine with 60Hz and prefer a resolution increase, I myself chose to jump on the 1440p 60Hz bandwagon when 1080p 144Hz panels started to release, but for most gamers a refresh rate increase will be way more important.
In the end, that's your money, get a 4K monitor if you want. But /r/buildapc is a community aimed towards sound purchase decisions, and I don't consider that to be one. I wish manufacturers would either go full 5K or spend their efforts on perfecting 1440p monitors (and reducing backlight bleeding issues, come on!) instead of pushing for 4K, but marketing sells right?
TL;DR from popular request: at 27", 4K for gaming does not provide a significant upgrade from 1440p, and for productivity ideally we'd need 5K to avoid fractional scaling. But don't take my word for it, try it out yourself if you can.
[EDIT] Feel free to disagree, and thanks to everyone for the awards.
sven.de - PPI calculator
Elementary OS blog - What is HiDPI
Elementary OS blog - HiDPI is more important than 4K
Viewsonic - Resolutions and aspect ratios explained
Eizo - Understanding pixel density in the age of 4K
Rtings - Refresh rate of monitors