Also, I think it's important to point out that if Apple were to release a 4'' iPhone, it would absolutely need to have all the groundbreaking features that the larger kin iPhones would have as well. I think the 5C (and its relatively little success and fanfare) proved that there is not the demand for underpowered, inferior iPhones. Just because you are using a smaller screen for personal preferences does not mean you should be willing to sacrifice performance a larger screen device might offer. I myself would strongly consider a move back to a 4'' phone should one be released, and I would be so, so disappointed if I didn't get all the same features people with the larger iPhones would get.
Another example of little demand was the original iPad Mini. Many, many users expressed their desire for a device of that size, however it was not until the iPad Mini 2 that it saw mainstream success because the iPad Mini 2 matched the iPad Air's component set at the time. While there is was strong demand for the original iPad Mini and iPhone 5C, it is not the same kind of strong demand Apple sees in other devices. Apple focuses on the best products it can make, and I don't think those products matched the company standards the company envisioned for them.
Some features, such as battery life, storage capacity, etc., could understandbly be inferior on a smaller device, and I'd be okay with that, but as far as new recognizable features, I think it'd need to be the same. While the 6+ offers a superior camera stabilization system (can't remember the name off the top of my head), I wouldn't consider that a groundbreaking difference or feature over my 6. But that's about as far as I think Apple should go in addressing the same features for a smaller iPhone.