Pre-orders and previews begin April 10, launches April 24.
Apple Seeds First OS X Mavericks 10.9.4 Beta to Developers
It is unclear what improvements the 10.9.4 update will bring to Mavericks, but it is likely to include bug fixes and stability enhancements. It is also unclear when the beta might be released to the public.
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.