You don't really need 4K at 27", even if your vision is very good you shouldn't sit that close to your monitor. 4K is awesome for large sizes, 32" and up, I would say.
4K at 27" would still be considerably lower pixel density (~160 ppi) than my old Motorola Atrix phone (275 ppi), which had lower density than my current HTC Rezound (342 ppi). I found the difference between the Rezound and the Atrix to be striking, so I think 4K at 27" would not be overkill by any means at half the density of the Rezound, even taking into account that the display will be a little farther from your eyes.
For larger displays - yes. For smaller - not really. After a certain density at a given viewing distance you stop seeing the difference, but doubling the density requires 4X more powerful hardware (and battery) to keep up.
