don't get your hopes up. From what I've seen, the new 4" model would entail moving the 5s innards into a plastic casing like the 5c, if not the exact same one. From a production standpoint, it makes total sense, because it would replace CNC-machined aluminum with stamped steel and plastic (cheaper and less time-consuming to produce), while reusing the current 5s components.
In much the same way that the 5c was a cost cutting and production capacity move, the "6c" would have a similar role. After all, the "6c" would likely occupy the "free phone" price point.
Ideally, I would like to see a 4" iPhone with the latest processors and hardware features. It would not only have very fast performance, but would also have true all-day battery life, since it would not have to power a large screen or process higher resolution. But, I doubt that Apple will go that route. They've already established the 4.7" screen size as their mainstream price point, and I doubt that they would carve out a lower price point just to offer a new 4" option with the latest and greatest features.