I am currently a proud owner of the Yamakasi Catleap Q270 27' inch 2560x1440 S-IPS monitor which is overclocked to 120hz.
I have a 2nd Dell U2410 1920x1200 H-IPS as my secondary monitor.
To me, working on word/excel documents and browsing the web I like the extra real estate. Improves on my productivity.
However text and words can seem very small, straining my eyes sometimes. 4k will make it even worse. So I am pretty against 4k which will destroy my eyes while reading text.
Gaming however is a different story. 4k really does improve the gaming experience. Graphics will look more crisp and sharp provided the game supports those high res textures. Because 4k will still make the game look like a blurry mess of textures if the textures are not high res.
If you ask me, I rather get a high refresh rate monitor than a 4k monitor. Because the difference between 60hz and 120hz is unreal. Movement really feels so much more silky smooth than 60hz.
This is also on the assumption that your PC is capable of supporting the 4k and/or 120hz. Because if your PC is not powerful enough to support, there is no/little point in discussing further.