iPhone OS 4.0 Beta is already leaked and in order to get it installed on your device, you need an Apple Developer account. There's a catch though. You can't simply sign up the developer account; you are required to pay USD 99. I don't think it's worth to pay that much for the sake of trying iPhone OS 4.0 Beta which will eventually come out around this summer. Fortunately, there is away to get around this problem (install iPhone 4.0 beta without UDID registration), thanks to ih8sn0w.
1. Decrypt your firmware files and get the rootfs.dmg file.
2. Open the rootfs.dmg file within the IPSW file.
3. Then navigate to System/Library/CoreServices and copy over SystemVersion.plist
4. Now open SystemVersion.plist
5. Remove the following two lines from it
6. Add it back into the rootfs.dmg file, overwriting the old SystemVersion.plist.
7. Now restore the new IPSW file to your device
Apple has finally released the iOS 4 for all the users. It is really a good news that Apple is giving the iOS 4 Firmware update for free for all supported iDevices. Users can connect their iDevice to their PC or Mac and launch iTunes 9.2 to update the firmware to iOS 4. But if you are facing any issues with the automatic firmware update you can try out the below procedure to manually update your iPhone or iPod Touch firmware to iOS 4.
