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 would only push iOS updates to those phones which they consider still lasting. By this standard, iPhone 4 was supported with the latest version of iOS until the release of iOS 8, which is in the middle of 2014. iPhone 4 was released in the middle of 2010. So we can draw the conclusion of: 4 years.