Hi, I like 4K displays but unless it is on a 50" display, I need to scale up the fonts to see the letters.
Also true of 5k displays - "raw" 5k mode would be unusable on a screen who's size in inches was less than your age in years. 5k Macs don't even let you select "raw 5k" mode without jumping through hoops - they default to the "looks like 2560x1440" HiDPI mode which scales up the system text, icons etc. by x2. The result is that, on a 27" display, the system text, icons etc. are the same size as on an old 1440p non-retina iMac or Thunderbolt/Cinema display, only much sharper.
Now, some 4k displays
will come up in "raw 4k" mode with tiny text, but the optimum mode would be "looks like 1920x1080" mode which, on a 21" screen, gives the same sized text as a pre-retina 21.5" iMac but again, much sharper. On a larger, 27" or so screen, the system text and icons in that mode start to look a bit big, and use a lot of screen "real estate" but they're still pin-sharp unless you have bionic vision.
Those modes are the optimum for both 4k and 5k respectively - most modern software will be rendering everything at the screen's "native" resolution and any old software or low-res bitmaps will be scaled by exactly 2x, which minimises any 'artefacts'.
Thing is, if you want to stick with those "optimum" modes you end up wanting to choose your screen resolution to match the display size.
So the sweet spots for MacOS are 21" @ 4k, 27" @ 5k, 32" @ 6k, and by moving up you're not adding sharpness, you're adding screen area.
(Those sweet spots are a bit of a Mac thing - Windows lets you freely choose the screen scaling, which sounds better than Mac but is very dependent on software behaving itself. I wouldn't bother with 5k on windows, and based on the number of 5k displays currently widely available - order-of-magnitude 0 - the world agrees with me).
So that's a problem because the only 27" 5k display currently available in most places (the LG one is widely unavailable) comes with a sometimes inconveniently attached iMac, while the only 6k display on the planet costs $6000 or $7000 if you can't find some bricks to prop it up on.
So, if you want to get an
affordable 27" or larger display, the options are:
(a) get that 50" 4k model that you can read in "raw 4k" mode.
(b) Get a smaller 4k and run it in one of the "non-optimum" scaled modes.
(b) is probably the answer if you've got a Mac with a half-decent GPU. The "non-optimum" scaled modes work by rendering to an internal buffer at twice the "looks like" resolution and then re-sampling to the native display resolution.
The result is very, very good - don't be put off by past experience of running a low-res LCD in non-native mode.
I have a 5k iMac and, as a second screen, a cheapo 28" 4k Dell. The colour reproduction on the Dell isn't a patch on the iMac (better 4k screens are available), but in terms of sharpness it is very good. I normally run the Dell in "looks like 2560x1440" mode which effectively means that the screen is rendered to 5k and then downsampled to 4k. Size/screen-estate wise, the result is the same as the 5k (well, 1" bigger) - The result is slightly "softer" than true 5k but most people with mortal eyesight would only worry if they looked unnaturally closely.