Apple has just released iOS 9.0 Beta 4 build 13A4305g, for Apple subscribers Developer Program
The fourth beta of the upcoming iOS 9 for iPhone, iPad, iPod and iPhone is available for download in IPSW images for selected devices directly from your Apps Developer Account Member Center
Users who are not enrolled in the Apple Developer Program will be able to update their devices in the coming days when Apple makes the update available via OTA (Over the Air) updater.
However, remember before installing the public beta you should know that it is a pre-release of the upcoming iOS 9 and not designed for devices you use.
So if you are interested in the stability of the device, it is advisable to avoid installing it.