Considering Apple won't make a super-retina display, a 4 inch iPhone would have a 640x1096 resolution
The time draws close for the next iPhone (iPhone 5 or the New iPhone?) to show it’s face, and each day more sources are suggesting that it will sport a 4-inch display instead of the current 3.5 inch. Considering it’s current width to be almost ideal, it might turn up to be a phone that’s simply a little bit higher, with all the other dimensions kept the same (well, maybe thinner too!).
So I did some lame-math and it turns out that if we add some pixels to the current 640×960 resolution, we’ll end up with roughly 1096×640. That’s still lower than the first two iPads, but it will add that dreadful fragmentation to the Apple line, and the apps will grow in size to accommodate three phone and two tablet resolutions.