Did you just update your iPhone 4S to Apple's iOS 5.1? If you did and if you're with AT&T, chances are you would have noticed your 3G signal icon magically transformed into 4G!
Fueled further by a barrage of tweets coming from excited iPhone users, the newly rolled out iOS version and AT&T by extension got a major boost in marketing. Everyone thought that with the 4G icon came improved performance and data transfer speed, not!
The icon change is purely cosmetic and does not, and will not offer performance upgrades or faster data transfer speed. The transformation merely fuels the network's efforts to market iPhone 4S as a 4G phone, possibly in an attempt to dissuade subscribers from T-Mobile which offers a couple of 4G-branded Android smartphones (ex: Samsung Galaxy SII) or simply putting it on top of Verizon's own 3G iPhone 4S.
Both Apple and AT&T may not be entirely wrong as HSPA and LTE are both called 4G by the industry.