There is something magical about iOS 7 on the iPhone 4. The flat design, the smooth parallax (well, mostly smooth), and that glass back. But then reality hits: You open the App Store to download Facebook Messenger, and you get the dreaded pop-up: “This app requires iOS 10.0 or later.”
If you are reading this on a computer, there is a good chance you just picked up your dusty iPhone 4, charged it via that chunky 30-pin cable, and watched the Apple logo glow to life.
The iPhone 4 on iOS 7.1.2 is a classic—like a vintage car. You can drive it to the corner store, but don't try to take it on the interstate. Messenger is the interstate.
You actually need to talk to people without your phone rebooting randomly.
If you search the App Store on the device itself, you will see "Get" or a cloud icon, but tapping it will trigger an error: “Unable to Download App. Messenger is not compatible with this iPhone.” However, Apple has a hidden feature for vintage devices. If you have previously downloaded Messenger on any iPhone using your Apple ID (even 10 years ago), you can exploit the "Last Compatible Version" trick.