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.
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.
And not just the camera difference on the 6/6+ but the battery also is amazing on the 6+ then the screen display. Battery lasting a long time for me is a big plus. That means I can simply do more for a longer period of time on my phone without charging it.