Buying an iPhone 4s at full retail, i.e. unlocked does not require you to sign a contract. If you add the line to a family plan, it will cost you $10/month plus the data plan. There is no discount for not signing a contract/bringing your own equipment. $20 for 300mb/month or $30 for 3GB/month.
For service or support questions including existing order status, call: Customer Support: 1-800-331-0500
Information on iPhone unlocking can be found here by copying this link into your browser: https://www.att.com/deviceunlock/client/en_US/