I would have thought the camera in the existing iPhone 4 was sophisticated enough to do facial recognition.
Its not as if the camera is a high percentage of the phone cost, but is a high percentage of why people buy premium phones.
2D recognition probably but not the new rumored 3D sensing likely incorporating infrared sensing as well that will be necessary for this to be secure. If the current camera is good enough then they could enhance the security of the older phones by requiring TouchID and FaceID together to authenticate.