You cannot replace a 4s with a 4 screen as the auto brightness will not work. On the 4s there is a little white dot to the right of the proximity sensor windows. This is the ambient light sensor. Beneath this white window is plastic prism in the main housing which transmits the light onto a sensor on the main circuit board on the other side of the chassis. The lcd itself without the chassis and glass on the 4s and 4 are interchangeable.