The need for a 4" phone exists now because there isn't enough of an incentive for a larger phone. Split screen single apps, longer battery life, and more screen real estate are a start, but those that are in love with their 4" phone need more to switch over and join the rest of us.
More RAM, larger phones, and the iPad introduction of three types of multitasking all hint at what we might see for iOS X and the iPhone 7. Not to mention interactive notifications and quickly returning to apps that we now have. If we don't see some combination of Slide Over, Split View, or Picture in Picture on the iPhone next year I'll be very disappointed.