Perhaps the key headline feature of the newly-released OS X Mavericks 10.9.3 is proper support for external 4K monitors. Previously, Mavericks would render text, icons and other user interface elements as-is, so everything appeared tiny due to the densely packed pixels on 4K monitors.
OS X 10.9.3 uses pixel-doubling to enable a true Retina experience where the size of the user interface does not change, it's just a lot sharper because OS X has a lot more pixels per square inch to work with.
Apple has now refreshed its support document with detailed information regarding compatible 4K monitors, display modes, video interfaces and Macs...