If you plug a 27" 4k display into a Mac, it will probably come up as "looks like 1080p" (1920x1080) as the default setting.
Because that's what it is. The Mac OS goes into "HiDPI mode", and automatically scales the display "by half" in both directions. The resulting image is VERY clear and retina-like.
If you want "smaller", you could set it to "looks like 1440p" (2560x1440) as xraydoc mentioned above. But be aware that the image will be slightly "less sharp" and that the scaling could affect the level of performance in some instances.
I'm thinking of replacing my old 27" 1080p display with a 27" 4k myself (probably Dell Ultrasharp). I've got older eyes, so the default of looks like 1080p will be good enough for me...