The new screen, 9to5Mac reports, will be 1136 x 640. The diagonal screen size will be just shy of 4-inches, at 3.999 inches, a width of 1.9632 inches and a height of 3.484. The key change, however, will be the aspect ratio, which will now be very close to 16:9 or a widescreen video.
Those extra pixels will be used, the report indicated, in iOS 6 for a modified home screen with a fifth row of icons and views of more content via extended application user interfaces using the extra 176 pixels.
As noted above, one of the earlier reports noted not just a 4-inch screen (3.999 inches or 4 inches; let's not quibble over 0.001 inches) but a changed aspect ratio. That report didn't have details on the exact change to the aspect ratio, but it did note that this change would make developers' lives a little adventurous for some time, as they would have to adapt their code to fit the new screen.
There was a theory that Apple might "letterbox" currently existing code, inserting black bands at the top and bottom of the screens, to reduce the impact on developers until they rewrite their code (does anyone remember the horrific 2X mode used for iPhone apps on the iPad until developers rewrote their apps with iPad specific versions?).
Naturally, developers concerned about this move should remember three things. When Apple changes things, it's because:
a) It's good for Apple
b) It's good for users (or at least, it's good for users in Apple's opinion)
c) It's good for Apple's bottom line.
a) and c) might be switched.
In other words, if this change is going to happen, Apple doesn't really care that much how it affects developers - or at least, third party developers.
Naturally, this is all still rumor. We won't know the truth until Apple spills the beans itself.