There are a few ways to get the Android O Developer Preview. The latest DP4 "includes the final system behaviors, the latest bug fixes and optimizations, and the final APIs (API level 26)", Google has said.
Since little is expected to change between Developer Preview 4 and the full release of Android O, developers can also use this build to test their app and ensure everything is functioning properly.
Speaking of, the beta release schedule for Android O has largely mirrored what Google did a year ago with Nougat. A new version of the Android Testing Support Library is also on the way with several new features to streamline application testing. They include notification channels and dots on app icons to alert you of notifications, picture-in-picture video playback, a streamlined settings menu, and more.
Dave Burke, VP of Engineering for the Android team at Google, notes that incremental updates to the SDK, tools and Android Emulator system images are set to arrive in the coming days.