It's certainly possible, especially considering there have been basically no leaks for parts nor the phone itself, unlike the 5c which was all-over the internet.
And that's the rub, either anything is possible, or nothing is possible, given the dearth of leaks.
There were also no leaks on the updated iPod Touch which more or less came out of the blue, which I would attribute to parts already in production for use on other products. Another thing, at least for the 6S based phone, I would expect that Apple has had the 4" version designed since before the 6 when they were deciding what direction to go. Probably prototyped it right up to before they went into production, which means all they have to do is flip the switch on manufacturing it with existing parts. The screen is really the major change to anything Apple is currently producing, but maybe they're burying it in their iPod Touch orders? And if they don't shrink the 5s derived phone, then changing the shape of the case edges and bezels is negligible too.