According to several graphs I found, the extra detail afforded by 4K resolution on a 27" screen will be fully visible if you sit closer than 3 feet from your screen. On a 24" screen it would be closer to 2 feet.
At my desk, my comfortable viewing distance is between 24-34", and I'd bet yours is similar, so assuming we both have 20/20 vision we would benefit from the extra real estate.
Any distance further than 3ft and you start to lose acuity for the extra pixels.
According to several graphs I found, you'll resolve the extra detail on a 27" screen if you sit closer than 3ft from your monitor. On a 24", you'd need to be a bit closer, but quite frankly my comfortable distance is around 2ft 10" at my dual-screen desk, and I'd bet yours is similar, so I think we both would fall in the category of still maintaining full visual acuity for 4K content on a desktop monitor.
That is, of course, assuming that we both have 20/20 vision!...