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/.
Apple's iPhone 6s was similarly declared the world's most-shipped smartphone in the second quarter of 2016 by research firm Strategy Analytics, ahead of the iPhone 7, and if IHS Markit's data is accurate, then Apple's last-generation smartphone remained most popular through to the end of last year.