I'd actually say just the opposite. The 4S may have the same form factor as the 4, but the internals are much different - to the point that the 4 is closer to the 3GS than it is to the 4S. The 5 will have a new form factor, but will likely have similar internals to the 4S.
Based on history, next unveil is usually a jump forward.
Current rumors, which are taken as such, suggest that the five may have a much stronger chip, possible be the first to upgrade to LTE, wear a new form factor similar to the Air. Less likely, but still rumored...a larger screen.
All signs point beyond a simple iteration.
The way Apple goes with the iPhone, then “S” variant, you can be pretty sure the next case will get a redesign. Apple has also started a trend where when the iPad gets a chip, the iPhone gets it next generation. Also, it turns out the Apple TV has a single-core A5, but done in a 32nm process, and that some newer iPad 2s have an A5 on 32nm. LTE is on the new iPad, but it’s still power-hungry and fed with the iPad’s huge battery.
Given all this, expect a redesigned iPhone 5 with at least an A5X on a 32nm process, and maybe with LTE if they acquire a less power-hungry chipset. For the case, rumor is that Apple’s exclusive license of LiquidMetal technology a couple years back, and the hiring of some metallurgy engineers, may finally show in a product.