Parallel to the first beta of iOS 8.4, Apple also started beta testing of OS X 10.10.4. This follows a few days after the release of OS X 10.10.3 and with supplemental update to fix slow boot-up.
What’s new OS X 10.10.4 (build 14E7f) actually brings is not yet known. Apple calls the Release Notes merely an “improved stability, compatibility and security.” The trial version is only available for registered developers and not for the participants of public beta testing.
OS X Yosemite 10.10.3 brought, among other things, the new Photos app, which iPhoto and Aperture replaced, and a number of new emoji.