Yes, that's the idea. And I base that on the "6s-" being slightly more advanced than the 6s to step it up closer to the 7 to take advantage of some new tech, without giving away any clues as to what the 7 will include.
I doubt it will have 3D Touch at this juncture, but it very well may have everything else the 6S has, including a 12mp camera. Maybe a better battery, a processor spec bump, improved radios, etc. This won't affect 6S customers who want the larger format, though it might make them more eager to upgrade to a 7.
And again, I suggest this because I feel that Apple needs to position this phone as flagship-adjacent phone to get the premium dollar from those who can afford to pay it. Otherwise, the customers who can afford more but want a 4" phone are going to cost Apple revenue. Meanwhile budget customers are going to be willing to pay more to upgrade their iPhones and keep the 4" size, so why not charge as much as possible, without discouraging an upgrade? I mean, This phone isn't designed to replace the 6S like the 5c was to replace the 5, as a budget phone -- at least it shouldn't, because Apple is just leaving money on the table. And I can't see Apple designing a brand new phone as an entry-level product, so that rules that out. Nor can it be a premium phone if it's going to be released mid-cycle, and mostly outdated 6 months later.