AT&T`s website advertises the iPhone 4S as running on a 4G which is obviously a mistake for the iPhone 4S features faster download speeds but it doesn’t support 4G as listed by AT&T.
If you check AT&T`s website, you`ll see a list of 4G devices, the top on the list is the iPhone 4S which is compatible with 3G at this point.
AT&T should work on fixing this mistake so it wouldn’t be confusing for people.
You can check out the image here.