I tried reading up on iPad 4's wifi speeds before I decided to order. From my layman's understanding, it's not just the speed of your Internet connection that determines the speeds you get on various hardware, including iPad. Basically, hardware limitations also can limit the speeds you achieve, which is why you can use an iPad and still get slower speeds than using a typical late-model laptop or PC on the same Internet connection.
iPad 4 offers something called channel bonding, which from my layman's understanding means the hardware itself moves data more efficiently, so that you get faster speeds on iPad 4 vs. all previous iPads, even on the same Internet connection.
If I'm wrong, I'd like to hear someone knowledgeable explain it to me better. But I doubt that Apple is selling an imaginary feature and falsely touting faster speeds, because those kind of results are easily tested and they probably aren't stupidly opening themselves up to false advertising claims.