download and install Instagram for iPhone4 ios 7.1.2

What happens when you try to download it via the App Store app on your phone ? It's possible that the app's developer doesn't support the downloading of older versions of the app (or not for iOS 7.1.2), in which case you won't be able to download it

Whether you can download an older version will depend upon whether the developer of the app supports you doing it. When you try to download it are you getting a message that you need to update the phone ? If you are the developer probably doesn't support downloading an older compatible version of their app