Hard to imagine Apple won't have a new 4" model of the best selling phone in the US.... ...even if it's mostly upgrading a few internals (e.g., A8, cam, whatever, and which would help 'splain why there've been no 4" model part leaks)...
...certainly would be a "bold" move however, given their intransigence on going bigger if they went ALL bigger in "one swell foop"...
...and they do confound me with their decisions some time....
I told a nurse at work that the iPhone size was increasing and she hated it. Made me realize that people still want small phones. The highest I would go is 6" but maybe for her it's only 4". The only solution is too bring out multiple sizes. You can't make everyone happy with only one size.
If they use the same internals but different resolutions depending on screen size to maintain the dpi I could see it but it would cause a performance difference; smaller phone would be faster. If you use the same resolution then the bigger iphone display would take take the hit compared to the smaller one. You can tell the difference between a 4.7" 720p display and a 5.5" 720p display.
Come September I bet people will not be happy either way it goes. It will be a very active forum.