Iphone 4 won't update past a 7.1.2

Solution 1:

The latest iOS version supported by iPhone 4 is 7.1.2. There is no official way to install iOS 8 or newer on older iDevices.

You can find a compatability list on Wikipedia.

Sorry but iOS 9 isn't supported on iPhone 4.

As you can see at the bottom of this page, the oldest supported iPhone is the iPhone 4s.

http://www.apple.com/ios/whats-new/

iOS 8 isn't even available for iPhone 4. As you can see here.

https://en.wikipedia.org/wiki/iOS_8#Supported_devices

The last supported OS is iOS 7.