How to adjust display settings on Mac mini?

well apple do not really produce their own LCD panels they buy them from the few manufacturers who do like most monitor makers do. any lg, Philips, Samsung, ... should work equally well if the right connectors are present in the monitor, and the things you wish like OLED or g-sync or what have you not is supported by said monitor

I believe that the OP's question is more about the quality difference from dropping from the 5K display on a 27" 5K Retina iMac (all 27" iMacs made since Late 2014) to a 27" 4K display.

Or about the quality difference in going from a 27" 2.5K display to a 27" 4K one, if they have one of the 27" iMacs released between Late 2009 and Late 2013.

I have an LG 27UP600 and am very happy with it. I have been using it for over two years now and has proven to be reliable and very good of my photo editing. FWIW, I previously was using a late 2013 27" iMac (2560x1440) and the two displays are quite comparable. The LG does have a matte vs. glossy screen but I have found that to be a non-issue.

Also, if you prefer the 2560x1440 display you may want to check out displays that are native 2560x1440 which can save you a few bucks and can be equally as good.

One thing to note is using a 4K display at 2560x1440 (referred to as 2K these days) is that the scaling will not be "pixel perfect". In the case of the 5K iMac, 2K is an exact scaling factor of 2. With a 4K display, that would be a scaling factor of 1.5. Some displays can work quite nicely with non-integer scaled video and some cannot. So, if you use the display at 2K, you may actually be better off getting a good quality 2K display.

My suggestion would be to go to a store and check out the displays in person and decide on the one that looks best to you and perhaps come back for a recommendation on whether the display plays well with Macs (the majority usually will).

I have a 27" iMAC. I also purchased a Mac mini and a 4k monitor. I was not happy with the 4k sharpness or brightness as compared to my iMac. I returned and bought a 27" LG 5k display as it had the picture plus camera, speakers and ports. Recently I also bought a Apple Studio display for comparison. Picture same as LG but speakers better however camera is worse.

The 27" 5K Studio Display costs almost as much as low-end 27" 5K Retina iMacs did. The difference is that those iMacs included not just the display, but an Apple keyboard, mouse, computer, and software.

Apple Magic Keyboards and Magic Mice also are not exactly inexpensive when purchased by themselves. You got them with 21.5" and 27" iMacs, and still get them with 24" iMacs – but not with Mac minis or Mac Studios.

The Mac Studio Display has a gorgeous 27" 5K display. Fits perfectly with a mac Mini.

True, it certainly does, but gosh it's pricey.