4 inches is amazing for a phone as described above. There is one catch, the iPhone today is really a mini tablet that just happens to take phone calls as well. This matters because 4 inches is amazing as a phone but is not good at all as a tablet. People want to use their iPhone more and more as a tablet these days. That's why Apple abandoned the 4 inch size.The benefits of the smaller form factor of a 4" are numerous:
Smaller display is less costly
Fewer pixels to fire up - longer battery time (even with less space)
Better heat dissipation
Less prone to bending
Actually being mobile,
Fitting a pocket (the best camera is the one you carry)
Better grip, no over-reaching your thumb
Does not make you look like an mobile addict, even if you are
So I would strongly advocate for going back to a human form factor. When I reach my alien 2,5 meters high and my hands are grown by 30 percent, I might just like the bigger size, but not now.
I think the 5/5S is a nice compromise size. Ok as a tablet (just) and not phablet sized (6/6+ family of phones) ie too large to be uaed properly as a phone.