It is unclear what improvements the 10.9.4 update will bring to Mavericks, but it is likely to include bug fixes, stability enhancements and better support on 4k dispaly. It is also unclear when the beta might be released to the public, which is weird because Apple wanted the public to test these builds out with developers.
The last Mavericks update, 10.9.3, added enhanced support for 4K displays and restored the ability to sync contacts and calendars between Mac and iOS devices over USB.
Requirements needed for the new release is to ensure you are on OS X Mavericks 13A598, 13A603, 13B42, 13C64, 13D65 or any previous 10.9.4 seed build. Also, very important please back-up important information or clone your drive before updating to any builds.