What are the effective scaling modes on OSX with a 4K display? Is an effective real estate of 2560x1440 possible? 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)
I bought one (the Dell P2415Q) so can offer the results...

Scaled options are:
1504x846, 1920x1080, 2304x1296, 2560x1440 and 3008x1692

That is to say, the macbook always outputs 3840x2160, 60Hz as far as the monitor is concerned. It just "looks like" those resolutions in terms of real-estate.

1920x1080 is the default, and other scaled resolutions are given with the warning that "Using a scaled resolution may affect performance".

eg.

Looks like 1920x1080:


Looks like 3008x1692:


Source: My MacBook Pro (Retina, 13-inch, Early 2015), running 10.10.4, 2.9 GHz Intel Core i5, Intel Iris Graphics 6100 1536 MB