Apple Seeds OS X Mavericks 10.9.4 Build 13E19 to Developers
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.