New Monitor - 2k or 4k?

HI everyone, my question is indirectly linked to COP: I am on the edge of buying a new monitor for photo editing with COP (only little vidoe work). It shall be a 27" item. Now I am not sure whether to choose a 2560x1440 or a 4k resolution.

What are your experiences and personal thoughts - is 2k enough or should I go for 4k directly? So far I have been using my 17,3" Notebook screen - so any solution will be a significant upgrade...

Thanks for your help!

Richard