Apple today seeded build 13E19 of OS X 10.9.4 to developers, just over a week after seeding the second OS X 10.9.4 beta and more than 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 and should be in the Mac Dev Center shortly.
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.
Update: The new build is now available in the Mac Dev Center.