The back button alone on iOS is usually upper left in Apple apps and 3rd party apps. This requires use of the entire screen (if right handed).
When I used competing mobile OS's I didn't have to use the opposite corner from where my hand was as often. Dedicated back buttons and menu buttons at the bottom and such. Obviously this wasn't always the case and depended on the device.
You can have a 6" screen if you only use the bottom 3" of the screen a majority of the time.
I think it would be nice most apps were like Safari that use the bottom because of back at the bottom and gestures. But apps like messages, settings, FmF, etc all require full screen use plus the majority of my 3rd party apps.
OSX utilizes gestures better and screen size is irrelevant.
Reaching across 4" is a stretch. Much bigger without a modification of iOS would be tough. At the very least a lot of people would need to adjust the phone in their hand quite frequently.
It's been already said, but I just tried the apps and all of them support the "slide onto the screen from the left"-gesture. Reaching any sort of button at the top left was never a hassle, again this is the advantage of having such a small screen.