How to downgrade iPhone 4 from iOS 7.1.2.

1. Jailbreak the iPhone with Pangu.
2. Open Cydia and add https://coolbooter.com to sources and install Coolbooter.
3. Choose an iOS version between iOS 6.0 to iOS 6.1.3 and allocate space for the OS.
4. After you have installed the desired iOS 6 version, you must install Safari Download Manager from cydia becouse Coolbooter has a bug that acts on safari in iOS 6. This bug disabel the ability for safari to open downloaded files in other apps. By installing Safari Download Manager you will get around this bug.
5. You have now dual boot, and after a reboot the phone gets back to iOS 7. For the phone to get to iOS 6 you must use the Coolboot app on the springboard.