How long is it safe to use iOS 7? I have an iPhone 4. I would like to know: since iOS 8 is not going to work on the iPhone 4, when should I be worried about the security of my device, since iOS 7 will eventually cease to be supported by most apps?
The important thing to know is that once an OS is no longer supported, it doesn't immediately become full of security problems. It's also not beyond Apple to give updates to non-current (for want of a better description) versions of iOS for major security problems. Therefore, it's not possible at this stage to say when iOS 7 will be insecure.