I recently invested in a gorgeous 4k screen which I drive with a 13" rMPB.
For the size of the panel and my personal preferences on font size, icons, menu items etc. a "scaled" resolution of 3008x1692 looks best. Unfortunately there's a performance penalty because that's not the "native" resolution of the screen or the "default" HiDPI version of the same. OS X tells you me as much when I select the resolution -- "Using a scaled resolution may affect performance".
My problem with the "default" option is that all the fonts, icons and menu items are way too big. In the "native" resolution everything is too small.
I can hit command-plus a few times in native mode to make a browser and most other programs readable, and similarly for command-minus in "default" mode. But the icons and menu bars don't change.
Is there a way to basically make everything (or most things) bigger or smaller so that I can have my goldilocks size without having to scale the resolution?