By the way: iPhone 4 supports iOS 7.1.2, released 4 years after the iPhone 4.
(I suppose there may be a security argument for upgrading as far as possible, but it's not like iOS 7 is up-to-date either. If you're concerned about security, you shouldn't be using an iPhone 4.)
Sure. But if you are making an app for say a government service that has as a requirement to reach 98% of mobile users, and you reach the goal if you count compatible devices (not people who choise not to upgrade) then it helps.