Niel Cybart, writing for Above Avalon:

The best way to understand why Apple is releasing a new 4-inch iPhone in 2016 is to look at the changing iPhone user base. Apple is now selling iPhones to an installed base of more than 550 million users with a multitude of wants, needs, and desires regarding their smartphones. Apple is making a bet that it is time to expand iPhone development to three different screen sizes in order to appeal to the 20% of the user base that prefer single-handed iPhone usage over larger screen options. Releasing a new 4-inch iPhone would be an admission by Apple that the only way to maintain a vibrant iPhone upgrade cycle is to expand the iPhone line.

Terrific analysis. Be sure to look at the iOS vs Android price pyramid graphic to understand the holes in the existing iPhone market coverage:

By targeting the premium segment of the market, Apple spent years developing a loyal iPhone base of hundreds of millions of users willing to spend an above average amount of money and time on their iPhones. This trend stood in stark contrast to the vast majority of smartphone sales taking place at the low-end of the market where consumers used their phones very differently.

Apple has long eschewed that lower end of the market. Will a 4-inch phone be inexpensive enough to help Apple expand to the lower levels of that pyramid?