Thanks to a smaller 32-nm die shrink of the A5 iPad 2 processor, the device consumes noticeably less power. MacRumors points out that the iPad2,4 has been used as a test bed for next generation chip processes. In contrast, the iPad 3 A5X continues to be produced using a 45-nm process.
The new iPad 2 models have identical performance characteristics, however they clock battery life of 15-30 percent longer than their 45-nm cousins. The iPad 2 is now available new from Apple for $399, although older 45-nm stock is still being sold.
According to AnandTech:
There's no known way to tell whether you're getting an iPad 2,4 vs. the older iPad 2,1 without opening the box. The 2,4 unit I ended up with was made in China, ruling out manufacturing region as a way of telling. The external box looks identical, as does the device itself.
There are still many of the original 45-nm iPad 2 models in stock, so there's no guarantee that purchasing an iPad 2 for $399 will net the new 32-nm A5 with improved battery life. We can only assume that Apple will move all iOS devices to the new 32-nm process in the near future, as the battery life benefits are evident.
If you have just purchased an iPad 2, it's easy to determine whether or not you have the iPad 2,1 or iPad 2,4 model. Apps including Battery Life Pro or Linpack will let you know which generation tablet you have.
Although it won't affect performance, the new A5 chip is actually 57 percent smaller than the 45-nm version. Apple will continue to seek more cost-effective manufacturing methods that simultaneously improve the battery life of its mobile devices.