Actually, I think the Samsung S4 will look completely different again. Samsung has a different (or no at all) design philosophy.
Apple thinks design should be as reduced as possible, this implies a hypothetical perfect design that has no redundant parts at all. So the designs of the iPhone "converge" in some way and will change less from iteration to iteration until – again hypothetically – the perfect design is reached. If they would just forcefully change the whole design concept they would either admit they were completely wrong before or that they have no idea what they're doing.
Samsung however understood that most people are not seriously interested in any design philosophy. Well people like to hear that there is a philosophy behind it, but they don't actually care. What they really want is something new and shiny. And if a new iteration of a popular phone series doesn't look different, people won't buy as they think its uninnovative (just look at the people here).
Personally, I would have been pretty disappointed if Apple had changed the design just for the sake of it. This tells me they might have a plan in some way and are not still just guessing wildly.
Well said. Look at Porsche slightly altering their iconic design that is decades old but is frequently praised for its engineering and brand evolution.