Apple updated OS X Mavericks to version 10.9.3 today, adding better support for 4K displays, sync improvements, and bundling Safari 7.0.3 (previously a standalone update) into the OS.
Improved 4K display support is available for the Late 2013 Mac Pro and 15-inch MacBook Pro with Retina Display, and it allows OS X to drive a connected 4K display with pixels in "2x" (Retina) mode for high-resolution graphics. As for sync improvements, OS X now allows contacts and calendars to be synced between a Mac and iOS using a USB connection.
Other improvements in OS X 10.9.3 are listed on Apple's official release note page. OS X 10.9.3 is available through Software Update on the Mac App Store or via direct download using the links below.