My only question is - why?
Are people really going to see that kind of difference? On a 4" phone?
Exactly. What is the user benefit? There is none that I can see.
Now if they are using some new screen where the touch points are in the pixels and thus doubling it would make the screens even more sensitive and we can write without looking like first graders etc, okay.
But doubling the pixels just to have more pixels is spec wanking and that's not Apple's style