Apple has just released iOS 9.0 Beta 4 build 13A4305g, for subscribers to 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 registered with the Apple Developer Program will be able to update their devices in the next few days when Apple will provide OTA (over the air) updater updates.
However, remember before installing the public beta you should know that it is a pre-release of the upcoming iOS 9 and is not designed for devices that usesth.
So if you are interested in the stability of the device, it is advisable to avoid installing it.