Apple today seeded build 13E16 of OS X 10.9.4 to developers, approximately two weeks after releasing the first OS X 10.9.4 beta and almost a month after the public release of OS X 10.9.3, which included enhanced support for 4K displays and syncing improvements.
The beta is available through the Software Update mechanism in the Mac App Store as well as through the Mac Dev Center.
It remains unclear what improvements the 10.9.4 update will bring to Mavericks, but it is likely to include bug fixes and stability enhancements. Apple is asking developers to focus on Graphics Drivers, Bluetooth, Audio Drivers, Finder, App Store, and Safari.
Apple has also seeded Safari 6.1.5 and OS X Server 3.2 to developers.