4K Scaling Issues with Mac Mini (2018)

I had the same issue with a new 2018 mini connected to a Sony 4K TV (native resolution 3840x2160). I could select the native resolution, but the UI was unscaled and so everything was microscopic even on a 50" screen. The problem is that macOS isn't enabling HiDPI resolution scaling. I don't know why that is, it is a 4K display after all, but the good news is that you can manually enable scaling using the terminal. Enter this command as an administrator user, then type your password (the cursor won't move while typing. Just type your password and hit enter):

sudo defaults write /Library/Preferences/com.apple.windowserver.plist DisplayResolutionEnabled -bool true

Reboot the mini and open System Preferences -> Displays. Hold the Option (alt) key and click the "Scaled" radio-button. You should see a number of new options, some of which will be marked "HiDPI". You may have to experiment a bit to find the best one, but for me "1920x1080 HiDPI" enabled the native 3840x2160 resolution and scaled the interface up to look as if it were 1920x1080.

Once you have found one that works for you, you can verify that you're getting the right resolution by clicking the Apple Menu in the top-left, selecting "About This Mac", then the "System Report" button, then clicking "Graphics/Displays" in the list on the left. This is what I see (I connect to the TV via an A/V receiver):

Displays:

AV Receiver:

Resolution: 3840x2160 (2160p 4K UHD - Ultra High Definition)

UI Looks like: 1920x1080 (1080p FHD - Full High Definition)

If you decide you want to revert the change above, just use this terminal command:

sudo defaults write /Library/Preferences/com.apple.windowserver.plist DisplayResolutionEnabled -bool false

The OP seems to have a good understanding of how 4K displays work. UI scaling is a feature of macOS designed to solve this exact problem - of text and UI widgets appearing too small on high resolution displays. It's been needed ever since Apple introduced so-called "retina" displays.

The issue is that the 2018 Mac Mini is failing to enable it when it should. I'm writing this message on a 2013 Mac Pro using a 27" 4K (3840 x 2160) display, which is running at the full native 4K resolution. The Mac enabled UI scaling without me having to do anything. This is what the "Displays" System Preferences pane looks like for this display:

The "Scaled" option allows you to choose how much the UI is scaled up. I choose to have the text at a larger size to make it easier to read. Modifying that option does not change the display resolution (verified using the monitor's on-screen menu) - it will always be 3840 x 2160. It just changes the pixel-size of the text, etc.

I also have an Apple LED Cinema Display attached to this machine. It's a 1920 x 1200 display, and so macOS has correctly not enabled UI scaling for that display:

With UI scaling off, the resolution of the display is modified using the "Scaled" option. The pixel-size of the UI remains constant.

To see what UI scaling actually does, check the two images above. The exact same dialog is 1560 pixels wide on the 4K display, but only 780 pixels wide on the Cinema Display.

So that's how it's supposed to work. There's nothing unusual about expecting to be able to use a 4K display comfortably at 4K resolution. It's the norm, and has been for many years in the Mac world.

Does your TV have a 4K upscaler that is activating on that 1080p HDMI input feed?

you are expecting too much. 4k on a 27 inch display or even a 32 will be too small. do some research. your 'happy medium' is what all the normal people choose.

The manufacturerer probably has downloadable PDF user guides for the specific model of TV, in addition to the specifications information on the product web page.