I think a 4" iPhone 6C is plausible. If Apple were to build it, I would be shocked if it were to not include Touch ID. I would be surprised if were to not include NFC, because it would be stupid to not include support for Apple Pay. That makes the A8 chip the logical choice, even if the camera and flash might be taken from the iPhone 5S.
3/30/15 at 11:36am