How do I install an older version of an app

IF the developer has opted to keep an older version, you can try and install it using these instructions - Install an app on an earlier version of iOS or OS X - Apple Support. However, it is entirely up to each developer as to what versions they choose to make available in the App Store. But if a developer chooses to pull their older versions of their software, then you cannot get the older release.

Also note that an iPhone 4 cannot be upgrade beyond iOS 7.1.2. To run apps requiring a newer version of iOS you would need to get a newer iPhone.

If you have previously installed/purchased an app, you can often re-install it again from your purchase history, even if that version is no longer current. If that old app will still work with online services today, that is a different issue.