If you genuinely see a
significant difference it. is. not. because. of. pixel. density. It's because of any number of other factors specific to your display and/or you. Such as a poor quality(or defective) matte layer and/or the physical size of elements on one display don't match the other(did you ever bother to check this?) and/or differences in brightness/contrast and/or incorrect/mismatched sharpness settings and/or your vision isn't what you think it is etc...
I guess it's possible for something like a really crappy matte layer to create a
significant difference, but I've never personally seen one that bad, and I've used dozens of brands over the years. But this is mostly besides the point and I am still essentially right.
If you run perfect 1440p@2x on the 5K display and equivalent 'looks like 1440p' scaling on a 4K display, MacOS renders the exact same amount of pixels
; 5120x2880. The only difference on the 4K display is that 25% of resolution is lost to downscaling.
The overwhelming majority of fonts you see in documents, apps, websites etc are all 'typical' sizes. Eg the letter 'e' on this page is ~15x18 pixels at 5K resolution. On a 4K display it's downscaled to ~11x13. The aliased edges of fonts(the part that determines the sharpness of text) are usually only a single pixel wide and the rest is the actual font body and empty space. So most of the additional resolution is literally only one or two extra pixels to the font body. The physical size of one pixel on a 4K display(assuming 27") is 155 microns(0.155mm). On a 5K display it's 116 microns(0.116mm). That's only a difference of 39 microns(0.039mm). So only 39 micron reduction in width to the aliasing edges and maybe 39-78 micron increase in density to the font body. The difference between 185 vs 218ppi is
even smaller than that.
So statements such as yours and those I quoted
here being due to pixel density are patently false. It doesn't matter if you have perfect human vision, and eye disorder or anything in-between, the laws of physics of the universe in which we currently exist do not allow them to be true. If the physical size of elements on both displays match, there is no
significant difference, unless you're running super tiny fonts everywhere all the time and sit ~25cm away like an idiot.
If you don't agree with any of this you can whip out a calculator and macro lens and check for yourself, or maybe try a 4K display that doesn't use wax paper as the cover layer.
@Lyoha please read conversations in their entirety before replying with the same incorrect assumptions at least two other people have already been corrected on.