My eyes find 4k too small to read well even on a 15" display. So unless you have very good eyesight, you will probably run 1920 x 1080 or similar resolution. P

My eyes find 4k too small to read well even on a 15" display. So unless you have very good eyesight, you will probably run 1920 x 1080 or similar resolution. Personally, I prefer to use a pair of 23" screens to get enough screen-space.