By default the 4" display iPhone is always going to be the least expensive of the line-up.
But the 4" is going to be starved for space, which means in some cases, it's not going to be able to physically contain all the latest bells and whistles of the flagship phone.
If entry level phone buyers want larger phones over smaller, or they want those cheap larger phones at the expense of the mid, and top-tier phones, then Apple might drop the 4" phone down to the bottom as the only offering.
Keeping the base phones at 16GB is also smart because it effectively adds $100 to the cost of each phone, as upgraders know the value of extra storage. Perhaps Apple will eventually price the memory upgrades at $50 increases rather than $100, giving them wider price differentiation between models and tiers.
First note that you're talking about carrier pricing, it's probably better to talk about ACTUAL prices, as what carriers charge is rather irrelevant (as you're paying a lot more over the years, it's just hidden in a whopper bill).
But, I guess my question is, why is the 4" going to be the cheapest? I know that's what they do with some devices, but it would seem to me the 4" and 4.7" could be a similar price if they have feature parity... or, to put it another way, if components cost similar amounts, this is more about size preference than one being an upgrade. For example, maybe the 6s Plus costs more with that screen, but it would be a downgrade for me, not an upgrade. I don't consider it a higher-end model (aside from a couple features).
And, we keep using 'entry level' language. Again, I'm fine if Apple offers a 6c or 7c type device, but what many of us here are asking for isn't about entry level or cost, it's a matter of size. And if Apple wants to simplify the product line, then IMO, drop the 4.7" and just have a 4" and 5.5". The 4.7" is kind of a silly size, trying to still be a phone, but really too big. The 5.5" makes sense, as people who buy that are expecting a phablet.
I've also argued elsewhere, that the 16 GB iPhone actually doesn't make sense beyond the spreadsheets. It's BAD for user experience, and so IMO, no it's not a good idea, AT ALL! The old Apple (you know, the successful one), put user-experience first. The 'new' Apple is riding on that success. If they forget that, they'll quickly not be as successful. And, while I'll agree that $50 price jumps might make a bit more sense, it ultimately isn't about the money at the low end. The idea of starting at 32 GB should be so that anyone who buys a premium smart-phone shouldn't be faced with immediately having to learn techniques to keep their device in some free space.
Features as important as 3D touch would be a deal breaker for alot of people.
Yes, if I had to make the decision, I'd do 100% feature parity between 4", 4.7" and 5.5" *Maybe* there is some excuse for something like the image stabilization if it just won't fit in the 4" or 4.7" for reasonable physical reasons. (That said, there is no reason Apple has to keep making these devices thinner anymore... they are thin enough.)
I expect that 3D touch will become important somewhere down the road, but currently it isn't a feature I care much about. But, yea, for the sake of feature parity, I'd include it. Even if it increased the thickness of the 4" a bit.
I was looking at it from Apple's/business point of view. I pointed out where Apple could cut corners, to preserve it's fat profit margins while still preserving the user experience. ... I'm very confident that Apple could come out with a 4" iphone with "inferior specs" to it's big brothers and it would run and work just as well.
I don't think they need to cut those corners to preserve profit margin. It's more about the up-sell. The problem is, Apple is venturing too much into putting up-sell over user-experience. That, IMO, is a HUGE mistake. The part costs are going to be nearly identical between a 4" and a 4.7". Maybe the 5.5" costs a bit more to make.
I'm also sure they could make an 'inferior' model that worked well... except that many of the crucial things aren't the CPU/GPU speed, but stuff like RAM, the camera, touch ID, NFC, etc. Those things WILL impact the user-experience in a BIG way.
It's probably cheaper and easier for Apple to just put a 6s into a 4" case.... the reason it *wouldn't* be the same would be for model differentiation if they decide to make a 'budget' model or step-pricing.
In other words, a non-feature-parity 4" is a marketing decision, not a technical or cost one.
Perhaps for flagship dorks here on the forum (I'm one, I admit) but you have to remember everyone keeps telling me nobody even wants 4" phones anymore. It is highly likely that the people that do just want the smaller phone don't care about things like 3D touch...which honestly is a fairly "meh" gimmick for right now.
For me personally I just want an updated iPhone 5s with Apple Pay and I'll be happy for a good long time.
Who is this everyone?
What I see in most forums (not just here) are people screaming for a 4" iPhone... and then a few trolls in there just causing trouble. I don't see anyone arguing why Apple shouldn't make one. And there seems to be plenty of demand.
IMO, this 'everyone' seems more like people saying all TVs need to be 4K, or 3D or VR is the big next thing, or laptops need to add touch-screens, etc. In other words, silliness.
re: 3D touch - I can see how eventually when all apps use it, AND people get used to it, that it will be a nearly required feature. But, yea, for now it's mostly a gimmick (just an eventually useful one). Think of it a bit like right-click on mice. (Or, I should say, like right-click on mice back when it was more just a few pro-apps that used it to access special features without going to the menus.)