From the Apple website: Siri on iPhone 4S lets you use your voice to send messages, schedule meetings, place phone calls, and more. Ask Siri to do things just by talking the way you talk. Siri understands what you say, knows what you mean, and even talks back. Siri is so easy to use and does so much, you’ll keep finding more and more ways to use it.
iPhone 4s also has a more powerful processor (allows faster app operation) and a better camera (8mp in 4s vs 5mp in 4) with a better sensor that also shoots video in 1080p resolution.