IIRC, it won't work on the iPhone 4, because Apple dropped support for it with iOS 8. So now the 4S is the at the bottom of the ladder, while the iPhone 4 only went up to 7.1.2.
The iPhone 4 does not support iOS 8 at all. You need to use another jailbreak tool that supports your device and iOS version, most likely iOS 7.1.2 if you are on the highest iOS version the iPhone 4 supports. Pangu has a jailbreak that supports the iPhone 4 and iOS 7.