Bad news for iPhone 4S users , iPhone 4S can’t disable 3G and work on 2G only . This option was a good option in iPhone 3GS and iPhone 4 as 2G saves battery life than 3G so you can’t save your battery life anymore by this option in iPhone 4S.
On the iPhone 3GS and iPhone 4, users are able to go Settings > General > Network and turn 3G on/off. Turning 3G on, according to Apple’s own admission “loads data faster, but may decrease battery life,” which, naturally is true. However, the 3G option isn’t available following the same path on an iPhone 4S.This option doesn't affect iPhone 4S a lot but it was just a good option, new battery life of iPhone 4S may be a reparation for this issue. Let us know what do you think?