What are the effective scaling modes on OSX with a 4K display? Is an effective real estate of 2560x1440 possible?
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