Um...
4 --> 4s: faster processor, Siri, better camera, new antenna design
4s --> 5: faster processor, faster graphics, 2x RAM, better camera, HD front camera, new antenna design, new form factor, thinner, lighter, bigger screen, and oh yeah, LTE.
I absolutely do not understand the repeated protestations on this forum that the 5 is a "minor upgrade." With the possible exception of 3GS --> 4, it's the biggest upgrade in the iPhone's history. And I say this as a 4s owner who does not yet have a 5 (but really wants one).
Now all of that said, whether it's worth the upgrade from a 4s is a personal decision depending on use and finances. For me, the bigger screen and lte alone do it, easily. But the 4s is a great phone and runs really well on iOS 6, so I can easily see it not being worth the cost for some people.