The major points for most people are that the 4S has a faster processor, Siri (voice commands/voice assistant), and a much improved camera. Basically if Siri and the camera aren't worth it for you, then the 4S isn't that much ahead of the 4.
Does the 4 have speech recognition at all? I don't use Siri very much, but I use the speech function in the keyboard all the time. Facebook, email, everything.
