I think some people in this thread are a little too optimistic about how long it will be supported. It has been a little under 4 years since the 4S was released, which was a powerful phone for its time. I remember saying at the time it would probably be supported for longer than any previous iPhones.
Now it runs very slow and can barely multitask (RAM limitations I bet, ring a bell for the iPhone 6 anyone?). I try to play a game on the phone and I can forget about being able to take a phone call. Oh and forget playing Apple Music while some apps are open.
So your iPhone 6 may still be supported in 4-5 years, but don't expect nearly the same experience you have today. I expect the 6+ to be discontinued much sooner, by next year at the latest. Apple seems to like slowing devices down after a couple years and you'll probably want a new one. Your 1 GB of RAM is gonna limit you more and more each year.