Preview 4 comes with final Android O APIs (API level 26), although that was already available with the developer preview 3, and includes various bug fixes and optimisations.

The code contains the stable version of the Android 26.0.0 Support Library, while incremental updates to the SDK, tools. We expect to see the Android Emulator system images in the next few days.

The final version is expected to launch by the end of the third quarter. The exact release date is yet unknown.

Android O Developer Preview 4 images can be found here