The biggest change is that users can natively run OS X in a pixel-doubling 4K mode, which Apple is tagging with its familiar "Retina" designation. Twitter user Khaos Tian was one of the first to notice the new options in the Displays menu of OS X System Preferences, which took a little digging (and stumbling) since the new features weren't announced by Apple.
Image Source: Twitter (@KhaosT)
Enabling the pixel-doubling mode makes content appear the same as it does on late-2013 MacBrook Pro models outfitted with Retina displays. To be clear, before this OS X update, Retina MacBook Pro and Mac Pro users can run an 4K external monitor, but the best they could do is have it configured at 4096x2160 with a 24Hz refresh rate (eww!), which would spread things out and make them appear abnormal. Alternately, they could run 3840x2160 at 30Hz.
The new option not only offers a new scaling option, it also allows for a 60Hz refresh rate. This eliminates the stutter that's present at 30Hz, which even effects moving the mouse cursor around the desktop.