In this sense, it'd be "spend good money on what separates you from the virtual". That is, of course, our I/O devices :)
Sure. But that doesn't get over the law of diminishing returns. It might not be 4K that, but would you buy 8K or 16K because "it's the monitor I'm looking at", when the only thing it does is waste CPU/GPU, resources, and battery -- for angular resolution lost to the human eye anyway?
Personal experience tells me that the resolution difference between 4k and lower resolutions (for the same size screens) are Very Noticeable on my 27 monitors. (I have one 4k and one 1440p.) Text is noticeably easier to read on the higher resolution monitor. I think this is primarily because we sit much closer to monitors than to TVs. (I don't think I can tell when my TV plays 4k vs 1080p, as I sit about eight feet away.)
If I had a 30" monitor with 8k resolution, I would probably not be able to tell a difference, unless I went looking very closely to it, so I don't think I'd spend the money on that. However, at 40" or 42", I suspect one would notice it at the distances one normally sits from a computer monitor. At that point, my reason for not buying it would be that I can't afford it. ;)
But! Do a bit of research. I sent back a 24" 1080p monitor and got a 21.5" one instead, because the pixel density was too low. And it always will be, but still there's tons of 24" monitors out there at 1080p because people think bigger is better.
Make sure you're going bigger and better in the right ways.
Having a 120Hz monitor feels like having a computer that keeps up.
(Provided your input devices are also fast enough.)
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.
I can definitely see the difference between 4K and Full HD on a 15".
This has cost me thousands and thousands of dollars.
My logic is that when I use a computer, it's the monitor that I'm looking at. I don't sit next to the case staring at RGB LEDs. I can't "experience" a processor in any aspect except speed, and that's been perfectly acceptable for a decade.
But I'll stare at the screen for 8 hours a day for work, and an additional few hours at home.
Spending money on improving the image quality of something I look at for 10+ hours a day is absolutely worthwhile!
This is also the advice I give people: Buy a bigger, better monitor than you planned, and make sacrifices elsewhere. A larger 4K monitor is a very visible upgrade in the literal sense versus, say, a few hundred more megahertz on the processor.