Just a week after the official release of OS X Yosemite 10.10.3 and days after the first beta of iOS 8.4, Apple has posted the first 10.10.4 beta for developers and testers, though it is has not been (and is not likely to be) made available to public beta testers until future builds are released. The sparse announcement of the new beta says only that the update focuses on "stability, compatibility, and security of your Mac," which suggests it will build on the improvements made in 10.10.3.
