I honestly can't see any argument against having a 4" that makes sense.
And I can't see any argument for a 4" screen that makes sense
I don't know if people here remember when the original iPhone came out, a lot of people criticized it because it was "too big". That because of this it would never succeed in Asian markets because in those markets people wanted smaller phones.
Anyway, I don't know what an iPhone will look like two or three years down the road. I just hope it will be a good product.