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 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 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 not designed for devices you use.
So if you are interested in the stability of the device, it is advisable to avoid installing it.