9to5Mac has a post with pics on what a 4" iPhone would look like. This seems to be the ideal size that many mobile fans are looking for. Personally, I would love to have a 4" iPhone. My aging eyes would not only appreciate it but I reckon the additional real estate would allow for more interesting mobileness.
Don't get me wrong. Even if Apple doesn't come out with a 4" Retina Display on the next iPhone, the one after that, or ever, I would still very content with the current 3.5" display on the iPhone 4 and the 4S.
The question is would Apple be increasing the resolution of this theoretical 4" display? Perhaps in the beginning, we might not see this. Apple could still retain the same 960 x640 resolution with lower DPI and still call it a Retina Display because Apple marketing will rationalize that since this iPhone has a larger screen, you'll be holding further away from you and you still would not be able to distinguish the pixels.
I recommending heading over there and check out their pics. I swipe one as you can see just to wet your appetite. There more pics and a whole post examining how Apple would go about making this happen. There is one portion of their speculation that I disagree with.
Care to figure out which one?