In iOS 5.1, Apple and AT&T decided to change the 3G logo to a 4G logo since the iPhone 4S runs on AT&T's HSPA+ network. No, the update did not give you 4G, it's just the logo that changed, your phone will still have the same speeds. Since there is no untethered Jailbreak for iOS 5.1 as of now, you can stay up to date by changing out your logo on iOS 5.0.1. The Cydia package '4G for iPhone 4' will swap out your 3G logo with the official iOS 4G logo. Now you can look cool like the rest of the bunch. Enable this in Winterboard and after a respring, the logo will fall right before your eyes. 4G for iPhone 4 can be found in the Cydia Store under the ModMyi Repo for FREE. Works on iPad too!