I purchased a 24-inch, 4K Dell monitor (P2415Q) and thought I would post my results. By way of background, my previous monitor was a low-end Dell 23-inch monitor with a resolution of 1920 x 1080.
I connected the new monitor to my Mac mini (I5/256/16) with an inexpensive HDMI cable. I reset the monitor to HDMI 2 and adjusted the brightness down but otherwise left everything at the default settings. I left the mini's display setting at "Default for Display" and the mini's System Report shows:
Resolution: 3840 x 2160 (2160p 4K UHD - Ultra High Definition)
UI Looks like: 1920 x 1080 @ 60 Hz
I tested the monitor with text documents, internet, and YouTube videos. Everything was sharp and clear--way better than with my old monitor. Display speed was quite good, although I do notice a very tiny difference between this monitor and the old one. The size of UI elements and text is exactly what I would want it to be.
Perhaps my only quibble is that the desktop appears a bit uneven when the background is a solid color and no apps are displayed.
Anyways, I couldn't be happier.
Do most of these monitors have speakers built in to them? I’d rather not have to buy a pair of external speakers just to hear some sound as it clutters the desk