Why you need to disable or turn 3G off on iPhone 4s? Well surely for keeping your battery life from fast drainage since this network consumes more battery power in daily basis.
However, since the iOS 5.1 Beta 3 is available for developers only, you are not able to practice this method yet -- if you are not a dev -- however once the official iOs firmware launched for public release, your turn to download it will come. Therefore follow what it appears on this picture:
And that is a now step to turn or disable 3G and save your battery life from a quicker drainage. Hope this post useful for you and see you again in another post.