Do you mean that when you compare your iMac 5K and LG 4K displays side by side you don't see any difference at all, under the same resolution? Sharpness, color, and all other qualities?
Colour, contrast etc. will depend on what model of 4k display you choose. Lets be realistic here - if you go shopping for displays for professional colour photo work you can spend the price of a Studio Display, or more, on a mere 2560x1440 display...
As for sharpness - guess what: 5k is sharper than 4k (given the same screen size), and if you do a close, side-by-side comparison you'll easily see it. The question (which was what I think
@usagora was saying) is whether that is going to be so noticeable to you in day-to-day use that it's worth spending twice the price of a decent 4k display just to get that magic 5k.
I have a cheap Dell 28" 4k next to my 5k iMac and - if I put them both into "looks like 2560x1440" (which is a misnomer) mode - the closest you'll get to the same resolution - "best for display" on the 5k and "scaled" on the 4k:
* The iMac blows the cheap TN-panel Dell away on colour and contrast. Try to contain the surprise.
* Small text etc. on the Dell is really good but slightly grey and "soft" compared to the 5k, which is black and pin-sharp by comparison - but at regular arms-length viewing distances the difference is very subtle and you're certainly not seeing pixels unless you have very good vision.
"Looks like 1920x1080" mode on the 4k - which for modern retina-compatible software means
no scaling artefacts is 100% usable. It's just that the system menu bars, icons, dialogs etc. are a bit larger than they would be in a perfect world - but certainly not unusably so, especially if you full-screen the app, and pretty irrelevant for the
content you are viewing since most applications will let you zoom, select font sizes etc. to taste. Or, there's native 4k 1:1 mode, in which the menus/icons/etc. are very small and fiddly, but which would be usable at 28" if my eyeballs were 20 years younger...
Remember, "looks like 2560x1440" on 4k is effectively a full 5k screen downsampled to 4k, so it still carries far more detail than you'd get on an
actual 2560x1440 screen - it's not like you're wasting your 4k by using it in 1440p mode. If you've ever used a standard-def display at anything other than it's native resolution using the display's built-in interpolation - just forget that experience, "scaled modes" on 4k are night and day better, and the display is always getting a true 4k signal. Yes, there's extra load on the GPU which might have been an issue on an old MacBook or Mini with Intel Integrated graphics, but really shouldn't bother a M1 Max.
Also - a 27" 4k display viewed from more than 21"
meets the original definition of retina in that the angular size of the pixels at that distance is less than the typical resolution of the mk1 eyeball. That's only a rule of thumb - so your mileage (and eyesight) may vary - but what it does mean is that going above 4k on a 27" desktop display takes you into a realm of rapidly diminishing returns.
So, yes, a 5k display is still the perfect resolution for a 27" MacOS display - but the price of that is now $1600, and a 4k display costing half that might be perfectly good for your requirements. Plus, there are all sorts of alternatives to consider - maybe a pair of 24" 4k displays, or an ultra-wide display, or a 32"+ 4k screen that you could use in 1:1 mode, or a 4k primary screen and a domestic OLED or QLED TV for viewing HDR content....