I don't know if face recognition and face detection are capable of the same thing but according to Apples web site the iPhone 4 & 4S have face detection.
From Apples web site.
http://www.apple.com/iphone/built-in-apps/camera.html
With new face detection, the iSight camera is smart enough to know whether you’re taking a portrait or a group shot. It focuses on the most prominent face in the frame and balances exposure across up to 10 faces. So you’ll definitely catch smiles. And maybe even a smirk or two.
From my  iPhone 4S 16GB Black