The equals sign means is equal to. It is telling us that one thing equates to another; that they are the same.
The iPhone 4 is the most recent iPhone to be discontinued from the iOS Update cycle. It did not receive the iOS 8 update, so it's final software version is iOS 7.1.2. You can update to this version through iTunes, or download it from an IPSW repository like this website: https://ipsw.me/.
Major versions of iOS are released annually. The current version, iOS 10, was released on September 13, 2016. It runs on the iPhone 5 and later, iPad (4th generation) and later, iPad Pro, iPad Mini 2 and later, and the 6th-generation iPod Touch.