A 4K image on a 27” screen comes out to 163 ppi.
That’s just slightly worse in terms of clarity to a 1080p image on a 13.3” laptop screen (165 ppi).
ppi is one measure of resolution, but a far more useful one when dealing with optics is
angular resolution. That means you have to take viewing distance into account.
The angular resolution of the human eye with "20/20 vision" (which represents
typical human vision, not
the best) is - by definition - 1 arc-minute (= about 0.0003 radians). (If I recall correctly the test is being able to see a 1 arc-minute gap in a "c" and distinguish it from an "o").
If you look at a 300 ppi screen from 11" away, the angular size of a pixel is (1/300")/11" = 0.0003 radians, so close to the limit of what a typical human can resolve. That was the most likely basis for the term "retina display" coined by Apple for the 326ppi display on the iPhone 4 (for which 11" is a reasonable minimum viewing distance). I.e. from more than 11" away, people with 20/20 vision will not be able to perceive 1-pixel size features on a 300ppi screen).
If you do the same calculation for a 27" "4k UHD" display (
there's a calculator here) then the "retina" distance comes out at about 21" - so if you typically watch your 27"display from twice as far away as you hold your iPhone 4, you should get a similar "retina display" experience without obvious pixelation.
The whole "retina" thing is very hand-wavy (for starters many people have better/worse than 20/20 vision - and how various eye conditions affect the result is somewhat more complicated than that) but really, beyond 21" viewing distance, increasing the resolution beyond 163 ppi leads to rapidly diminishing returns. That doesn't mean you won't see the difference between 4k and 5k in a side-by-side comparison, but it should make you question whether the "improvement" is worth paying 3x or more extra for 5k.
(And, while we're at it, a "looks like 1440p" image on a 4k display is effectively a 5k image downsampled to 4k and contains a
lot more detail than you'd see on an actual 1440p display - although you do need to ask yourself whether your game/3D package can render smooth motion at 4k, let alone, 5k - which is also why you shouldn't be jonesing for a 120Hz 5k display until we've seen another iteration or two of Apple Silicon GPUs).