What AT&T and Apple have done is fairly pernicious: They've taken a phone whose network they once labeled "3G" and are now calling it "4G," even though nothing has changed. Your iPhone 4S is just as fast--or slow--as it was yesterday, before it told you it was on a 4G network.
Worse, they made the "update" to "4G" on the iPhone on the same day they launched a product that uses their real 4G network, making it doubly confusing. The new iPad and the iPhone 4S are both "4G"! So, they must be the same thing, right? Nope.
I don't want to get toooo boring--I explained everything about "4G" networks here, if you want a longer read--but basically, the new iPad and the iPhone 4S run on different networks. The new iPad works on this network called LTE (which is a horrible, dumb name, yes) and it's actually stupid fast. A real generation ahead of the mobile speeds you're used to, if you've got an iPhone or say, most other phones except for some of the very newest Android phones on Verizon and AT&T. Fast like your home internet fast. Apple claims up to 72 megabits per second. You'll never see speeds like that, but it's real 4G that you can feel.
The iPhone 4S, and most other AT&T phones, run on this other network, HPSA+. It's slightly faster than AT&T's older 3G network--theoretically, you can reach speeds of 14.4 megabits per second on an iPhone 4S. So it's faster than what's possible on an iPhone 4 or 3G, yes. But not much faster, in practice. It's not a generation ahead. It's certainly not on the same level as the iPad. It's not 4G. But AT&T is calling it that. And a whole lot of people are buying it.