@graeme leggett
The iPhone 4 is restricted to iOS 7, but it was introduced five years ago. Even then, if Apple feels it is a serious hole they may add the fix to an earlier version, as they did with the "goto fail" bug with an out of band patch for iOS 6 in March 2014, nearly SIX years after the 3gs that was affected was released. I don't think this particular bug rises to that level.
No one supports operating systems forever, the five years of support Apple seems to provide for iPhone is pretty darn good - way better than Google or Microsoft.
Biting the hand that feeds IT
