Following its usual months-long testing process, Apple released OS X version 10.9.3
to the general public today. The third major update to the operating system provides the usual blend of security patches, bug fixes, new (and restored) features, and future-proofing enhancements.
As was discovered shortly after the first 10.9.3 beta was released, the new update improves 4K display support
specifically on the 2013 Mac Pro and the 15-inch 2013 Retina MacBook Pro. When those computers are connected to a 4K display, they should be able to display images in OS X's HiDPI "Retina" scaling mode by default, and they should support the faster 60Hz refresh rate on compatible monitors. "Retina" mode will make on-screen images larger and sharper, while the refresh rate increase will make UI animations, videos, and games look and feel smoother and more responsive (provided the GPU is capable of rendering them at 60 frames per second in the first place).
Though the 13-inch 2013 Retina MacBook Pro has the hardware it needs to drive similar 4K displays—a Thunderbolt 2 port with DisplayPort 1.2 support and one of Intel's Iris 5100 GPUs—that specific computer is not mentioned in Apple's release notes.