A new version of Mavericks 10.9.3 is ready for developers to download at this hour. Late on Thursday Apple posted the new release to its Mac Developer Center. The new build is 13D17.
OS X 10.9.2 was released in late February with a fix for an SSL vulnerability, support for FaceTime Audio, Mail improvements and other enhancements, and the first 10.9.3 developer build was released one week ago.
Apple's release notes for the build are under non-disclosure agreement (NDA) with registered developers, but it's been widely reported that one of the changes involves improved support for 4K displays.
In any case, if you're a registered Mac developer, you can go to Apple's developer center and download the new build now.