So yeah, we haven’t dug ourselves out of this hole yet.
An average 4K TV at a typical viewing distance is going to be impossible to distinguish the pixels. Same with a typical 4K monitor, depending on the size and viewing distance.
EDIT: A 27" 4K display at 60 cm viewing distance (53° horizontal FoV) has 73 pixels per degree (ppd).
A Retina equivalent display is anything above 60 ppd (i.e. 1 arcminute per pixel) as far as I can tell.
Calculator here: https://qasimk.io/screen-ppd/
Quality desktop monitors viewed at their typical distance do not look pixelated, e.g. you rarely are able to see individual pixels especially if proper antialiasing is used, but they do still almost always have a lack of sharpness that could be corrected with a denser display.
On retina smartphones, making the pixels denser wouldn't make the image look any sharper from a typical viewing distance. Once you achieve that, I'd say it's "retina-grade".
If that were true, there would be no reason for Apple to switch from 326ppi to whatever they use now for new phones. The real point where this stops making a difference is likely somewhere much higher, just like with framerates. That doesn't mesh well with their luxury electronics marketing narrative, though.
Now of course, maybe the new retina screens appear slightly sharper if you have exceptionally good eyesight, but I have long ago stopped seeing any improvement in sharpness of smart phone displays. The screens do look better in terms of better colours, contrast etc, but at a typical distance with an individual with normal eyesight, adding more pixels does pretty much nothing at this point.
For programming work I'm currently using one of those ultrawide 3440x1440 monitors, which is just a wider version of 1440p (which is 2560x1440). My ideal monitor at the moment would be this 3440x1440 monitor but with a pixel ratio of 2. This should be relatively feasible for reasonably-spec'd machines to drive and to manufacture (it would still be significantly fewer pixels than an 8K display), but AFAIK nothing like this exists on the market.
I've been using one for a year. It would be hard to use anything else.
So the market for these is limited to people with desktop workstations or pretty high-end laptops. Which is probably why they aren't going anywhere.
I bet you were trying to use HDMI or an adapter? If so that's a known issue with the transport mechanism, not the laptop. My '17 MBP can do two 4k external displays at 60Hz on top of the internal display using displayport, so yours can definitely do one.
This is a good reference. Note that it's from 2016: https://9to5mac.com/2016/11/17/macbook-pro-external-display-...
So that's a 1080p widescreen at a pixel ratio of 2, and maybe it's because my eyes aren't as young as they were, but that's perfect for me. Certainly renders any font I throw at it as crisp as you please.
It's the MSI Prestige, if you're curious.
To get UI elements the right physical size for a 27” 4K screen it would render to a 5120x2880 (i.e. 2560x1440@2x) framebuffer then do a non-integer scale back down to 4K. Sharper than 2560x1440 to be sure, but you can notice the blurring it introduces.
Basically all large hi-DPI monitors for Macs are a compromise except the Pro Display, which is like $5000.
The 24UD58-B isn't quite as good, but for $300, it's a lot cheaper than $700.
I have 4k monitors at 27 inch because I prefer the magnification faction, but I can see the individual pixels.
Given the cost of these things, it might be more feasible to just sit further away and use a bunch of the 4K ones.