With the New iPad out, the iPad 2 recently experienced a price-cut on its models, with the lowest 16GB WiFi iPad 2 available for $399. And therein lies the difference between models: their connectivity. The 1st type of iPad 2 is WiFi only (iPad 2,1), the second is WiFi and GSM (iPad 2,2), while the third is WiFi and CDMA (iPad 2,3). Which brings us to the topic at hand: the fourth-type, known as "iPad 2,4".
So what's the difference in connectivity? Like the 1st iteration, the iPad 2,4 is WiFi only. It's specialty, however, lies in the SoC (System on a Chip); in the iPad's case, it's more commonly known as the A5 chip, or the processor. Previous iPad 2's all had the same 45nm A5 SoC, while our new contender uses a 32nm A5 SoC.
How does this affect you? A 32nm processor is going to be more efficient overall than a 45nm processor. The end result is that owners will get (according to AnandTech) slightly better battery life.
The only downside is that there's no real way to tell outside of the box whether you're getting one of these new models. Once you open up the box, however, you can run a benchmark test or a battery test to see the specs of your iPad. So if you're going to purchase an iPad 2 sometime soon, start hoping you'll get the better end of this bargain.