Can I use an AT&T sim card in my sprint iPhone 4s?

Well it would seem that your iPhone is locked to Sprint and I believe the only carrier who now supports authorised unlocking is AT&T, so it would appear that no, you cannot use your AT&T sim in your Sprint iPhone. http://support.apple.com/kb/HT1937

The long answer is no.

If and when Sprint ever unlocks the GSM portion of the phone, it will only allow use of a non-USA SIM card for travel.