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.
Solution 2:
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.