I think it all really depends on how Apple does it. If they stick with a 4" and 4.8" screen or go S/M/L by still keeping a 3.5" screen.
There is one real easy way they could do all this and thats go with a @4x image size. To me this seems like the most logical. If they do @4x and double the screen res. of 640x960 you get 1080x1920. So 1080p on a 4.8" screen.
Doing that keeps things very simple for developers.
lol 4x res for a .8" change? no