I have a 2015 retina macbook pro, the default scaled resolution of 1920x1080 would be too small for me. I currently have a 2560x1440 display and couldn't accept less real estate.
Under the display settings, if you select "scaled" you should be able to choose between "larger text" or "more space". If I understand correctly, all of these options should use 3840x2160 pixels to render the output (ie, the monitor wont know the difference). The default scaling option should be the equivalent of 1920x1080. I was wondering if you choose "more space" it would allow for the equivalent of 2560x1440 (but scaled so should look crisper than a 2560x1440 display).
As a bonus question, would this be comfortable on a 24" display (eg, Dell P2415Q)