4" is right up Apples alley. 4.3" is perfect for my hands, but not really the "middle" point. 4" is something that Apple can use to keep the display still technically a "retina display" while keeping a very similar form factor (height, width, weight) and also giving us all the screen spec bump we've been waiting for.
Of course, this is all speculation. Maybe it's me getting my hopes up. Worst case scenario, if Apple doesn't pull through Samsung will be getting my business.