Mac Mini M4 external display quality vs. iMac

If you want a monitor with roughly the same Retina-level PPI as the discontinued 27" Intel 5K Retina iMacs, or the current 24" 4.5K Retina iMacs, you would want something like

• A 27" 5K (5120x2880 pixel) monitor
• A 32" 6K (6016x3384 pixel) monitor

Monitors with these pixel densities include

• The now-discontinued first-generation 27" Apple Studio Display.
• The new second-generation 27" Apple Studio Display and the new 27" Apple Studio Display XDR
• The now-discontinued 32" Apple Pro Display XDR

There are also now 27" 5K (5120x2880) pixel monitors from several other vendors, including LG, BenQ, Samsung, Asus, and ViewSonic. Plus, perhaps, one or two 32" third-party 6K monitors.

There doesn't seem to be much choice in smaller screen sizes. There is a low-end $300 LG 24" 4K display whose specifications don't mention sRGB coverage, and a high-end Asus 24" 4K display with color accuracy features that might appeal to professional photographers and videographers, but whose price is something like $1300. There's a very noticeable dearth of 24" 4K displays in spite of the fact that UHD 4K is a common resolution for TV sets.

Note that the old 27" Thunderbolt Display is a 2.5K (2560x1440) pixel monitor. It is not as sharp as a 27" 5K Retina iMac display or an Apple Studio Display. On top of that, modern versions of macOS no longer do some of the tricks that older versions of macOS used to do to make text look good on that display. (Once you have a high enough PPI thaat you can draw letter shapes accurately without those tricks, the tricks become less necessary.)

Likewise, 27" iMacs started out with 2560x1440 displays. They got Retina 5K displays starting in Late 2014.