If you are out in the market looking for a 4K monitor, I think this article will help you make your buying decision easy.
I am using a 4K external display with my MacBook Pro. I used 4K resolution on different screen sizes, on a 24-inch, 32-inch, and a 43-inch.
So, the question,
Can you utilize 4K on a 24-inch screen?
The short answer is, No. The longer answer is mentioned below.
When I was looking for a 4K monitor with a 24-inch size, one monitor is available on Amazon India, and two monitors were available on Amazon.com.
The 24-inch 4K monitor is from LG, costs less than $800 and has got nice ratings by the buyers. But I was surprised to read the reviews that none of the reviews mentioned the text size when using 4K resolution on the 24-inch monitor.
But mostly all the people who have bought the 24-inch 4K monitor are happy with its problem. They use scaled resolution on the monitor, as they are using it as a secondary monitor to their laptops (mostly MacBooks). It probably gives them a feel of the retina display.
4K (3840×2160) on a 24-inch screen is too small. 4K gives 4x the screen real-estate of Full HD, which means 4K on a 24-inch screen would be like Full HD on a 6-inch screen.
If you want to feel how UHD 4K would look like on a 24-inch monitor, and you have a regular or non 4K 24-inch monitor and a MacBook, you can use this utility for Mac OS called SwitchResX, it will unlock many different resolutions for any monitor.
I have a non 4K 24-inch monitor, and I tried 4K resolution on it, the text was unreadable. Currently, I am using 2560×1440, which is qHD or 2K. The text at this resolution is a little brother but is still readable, and I only use it when I am working with multiple Windows in I want more screen real state, and there is not much to do with the reading of the text.
So my recommendation would be to keep the 24-inch 4K monitor if you are looking for a monitor to look at the text for a long time. My recommendation would be to get at least a 27-inch monitor with UHD resolution since the text on it will be equivalent to a 13-14-inch monitor with full HD resolution.