Funny that many ppl in this thread say 4" is too small and 4.2" or 4.3" is perfect. I remember seeing a lot of ppl saying 4" would be perfect before iPhone 5 came out.
In my opinion if Apple does want to release a larger phone, it should either go around or above 5" or just stay at 4", having anything in-between, Apple is still smaller than competitors and developers have to update their app again just to accommodate the extra 0.2,0.3 inches, which in reality isn't that much of a difference. I recently compared an Galaxy S2(4.3") with an iPhone 5, the S2 was a tad bigger but certainly not ground breaking difference.
That being said, I still think Apple will not release a bigger phone anytime soon. Even if we see leaked news/pictures, it just means Apple is testing various sizes and models like they always been doing. Sure a lot of people chose Android because of the bigger screen size, but I bet vast majority of them got Android is actually because of the price.
For ppl new to the smartphone world, iOS/Android ecosystem pretty much means nothing if all they care was screensize, then they would probably choose Android based on price, give that screensize is the same.