Apple is reportedly planning on releasing a new, smaller iPhone next year. This year's 5.5-inch iPhone 6 Plus and 5-inch iPhone 6 should have similar sized successors but with a smaller sibling.
According to MobileWorldLive Apple has seen high sales of its 4-inch iPhone 5, iPhone 5S and iPhone 5C even though the larger iPhone 6 and iPhone 6 Plus are the current flagship handsets. This is being taken to mean there are people out there who still prefer the smaller 4-inch display size that Apple has provided for years.
Of course it could be argued that rather than a preference for smaller screen sizes the popularity of these handsets could be due to affordable pricing. So if Apple does release a top spec 4-inch handset in 2015 it may end up being less popular if its as expensive as the other flagship handsets. That said we doubt Apple will ever make a phone it has trouble selling.
The 4-inch iPhone for 2015 is rumoured to feature top end specs including NFC for Apple Pay and Apple's fingerprint Touch ID sensor in the home button.
Apple should release the Apple Watch early in 2015 which may see the company announce this new handset at the same event. Failing that we'd expect to see it appear alongside the iPhone 6S or 7 towards the end of summer 2015.