It’s been one week since Apple seeded the first beta of OS X 10.9.3, which includes an enhanced 4k-display support. Apple has seeded a second beta of 10.9.3 asking Mac developers to focus on audio and graphic drivers, which were the same areas of focus when the first beta was released last week. Many users later discovered that the feature turned out to be a new pixel-doubling scaling option that can display a desktop at native "Retina" levels over HDMI.

The current OS X 10.9.2 which was released in February doesn’t support pixel- doubling. This particular issue limited Retina MacBook Pro users to a max scaled output resolution of 4,096 x 2,160 pixels at 24Hz. Now that there are new scaling options, Retina MacBook Pro owners can output to a 4K display at 60Hz.

The second 10.9.3 beta is available for registered Mac developers either through the Software Update tab in the App Store or as a download in Apple’s online developer in the Mac section.

Source: Apple