Can't install Find My iPhone app on iPhone 4 as it requires iOS 8
When attempting to download an app that has been updated and no longer supports an older version of iOS, you will receive the following prompt:

Download an older version of this app?
The current version requires iOS 8.0 or later, but you can download the last compatible version.
Cancel | Download

Tapping Download will download the last version of the app compatible with the version of iOS you are running.