Apple has just released the iOS 9.0 Beta 4 build 13A4305g, for those registered with the Apple Developer Program.
The fourth Beta of the upcoming iOS 9 for Appliances iPhone, iPad, iPod and is available for download in IPSW images for selected devices directly from the Member Center of your Apple Developer account
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 information 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 care about the stability of the device it would be better to avoid it installation.