The OS X 10.9.3 beta, released earlier today, allows for pixel-doubling, which makes content appear as it does on the late-2013 MacBook Pro with Retina display screen while maintaining sharpness. For most users, this new feature will make 4K displays easier to work with on a day-to-day basis.
With Apple's current external monitor support in OS X 10.9.2, Retina MacBook Pro and Mac Pro users are limited to a max scaled output resolution of 4,096 pixels-by-2,160 pixels at 24Hz, meaning system-level graphical assets are spread out. For example, the menu bar on the laptop would show up incredibly small on a connected 4K display.
Another output option for the laptop is 3,840 pixels-by-2,160 pixels at 30Hz, though the resulting image is similar and nowhere near native. In both cases, 4K signals are piped through HDMI.