Does FaceTime on iPhone 4s cost money?
FaceTime is an application that was developed by Apple and it does not cost you anything on iPhone 4S. This is a video calling software. You can get more details about it on http://en.wikipedia.org/wiki/FaceTime.
You must be connected to WiFi though to use it. Download Skype (free), you don't need to be connected to WiFi to use it and like FaceTime, it's free.