A new iPhone with a sub-4" display is about as likely as that voiceless, cellular data-only iPhone you wanted a few years back.
Smartphones are primarily used to consume and share rich media. Small screens are barely adequate for the vast majority of people, young or old. Ever see an older individual struggle to read text on an iPhone 4? Seen those cheap flip phones designed specifically for old people with enormous number pads? Bold/Larger text in iOS 7 was a godsend, but it's not enough. At least one of my friends who bought an iPhone 6 Pluse is using it in "zoomed" mode. He's in his late 20s, but he's red/green colorblind. The blown-up detail helps he discern colors that might be obvious to us, but borderline to him.
The difference between 3.5" and 4" iPhone displays is minimal. It's not worth opining. Be glad the 4" display is alive and well for another year. Be glad again when Paul's prediction is true and Apple releases an updated 'premium' 4" next year. Be even more glad when they shave off some extra fat by eschewing bilateral symmetry.
As far as naming goes, I doubt they ever officially append calendar years to product names. The two low-end iPhones being sold today would be both be "2013" models and still need additional designations. I see the next round along these lines:
iPhone 5C (possible silent update to 5S level innards)
iPhone 6 mini (4" screen, A8 processor, 16/32/64GB storage, new colors)
iPhone 6S (4.7", A9 processor, 32/64/128GB storage, new colors)
iPhone 6S Plus (5.5", A9 processor, 32/64/128GB storage, new colors)
It's ugly, but not really worse than iPhone Plus 2015, iPhone 2015, iPhone mini 2015 and iPhone 2014/2013. It's still better than "Samsung Galaxy S II Epic 4G Touch by Sprint."