First, are you sure it is a 4S and not an iPhone 4? The iPhone 4 does not have Siri.
Second, do you have a profile (Settings > General > Profile)? If so, the people who set up your profile may have disallowed Siri.
Third, is Siri restricted (Settings > General > Restrictions)?
Fourth, I believe Siri is not allowed in certain regions or countries. Might this be the reason?