It's all about the pixels nowadays and 4K displays are popping up everywhere. Apple's new OS X 10.9.3 update beta shows new support for 4K displays, along with a special retina mode, which doubles the amount of pixels you see, while maintaining and improving your display's sharpness.
The beta shows new scaling options for displays, so that 4K and other high-resolution displays not only get better support, but receive preference over lower resolution displays. The new scaling option allows for HiDPI scaling in 4K displays. The new pixel-doubling retina mode allows desktops to display at native retina levels via HDMI and Thunderbolt. The new options are visible in the OS X System Preferences section under the displays menu heading.
The current version of OS X Mavericks, 10.9.2, only supports a scaled output of 4,096 x 2,160 pixels at 24Hz. At those levels, the performance of your 4K display is not optimized, but with the new output levels for 4K displays hitting 60Hz, things should start looking a lot better. The update will actually double the current refresh rate of 30Hz, which will not only improve the crispness of your 4K display, it will also ensure a more smooth, seamless performance while editing 4K video. For the most part, this update will improve the lives of filmmakers using OS X Mavericks.
Currently, the updated scaling options will only work on the 2013 Retina MacBook Pro laptops and the 2013 Mac Pro. The iMacs, other MacBooks and old Mac Pro desktops don't have a DisplayPort 1.2, which is necessary for the new scaling options to work. Still, the people who really will benefit from the improved support for 4K displays, probably already own the new Mac Pro or at the very least, a retina Macbook Pro from 2013.
Apple is encouraging developers to use the OS X 10.9.3 beta to improve graphics and audio performance. The most recent update to OS X10.9.2 was just a simple security patch, which was necessitated by the SSL "gotofail" issue. Now that everyone's Macs are secure again, Apple wants to get back to improving more performance-related issues.
The new Mac Pro is a very big deal for Apple and the company wants to maintain its reputation of being the go-to manufacturer for artists, filmmakers and musicians. The Mac Pro's hardware is certainly more than up to snuff, but its software has left something to be desired for those working with 4K displays. Apple hopes to change that with the upcoming OS X 10.9.3 update.