Upgrade iPhone 4 to run iOS 9
The latest version of iOS that can be installed on iPhone 4 is iOS 7.1.2. It is not possible to upgrade it to run iOS 9.

iPhone is pretty old now as per iOS release schedule. It was discontinued in 2013 with support being stopped by Apple in 2014. To run iOS 9, you need at least an iPhone 4s which can run iOS 9.3.5.