Apple’s interface design in macOS is set up so it is comfortable for most people at a density of about 110 pixels per inch for non-Retina, and about 220 pixels per inch for Retina — text is readable and button targets are easy to hit at a normal viewing distance. Using a display that isn’t close to 110PPI or 220PPI means text and interface elements will either be too big, or too small.
The trick is not to focus in the screen size and assume a 4K resolution is good enough. For true retina you need 4K on a 21.5in or 5k on a 27in. Otherwise it’s better to go for a non-retina 27in at 2560x1440.