The Bumper is not an ideal match for the iPhone 4s. The buttons volume up / down do not really fit into the recesses of the bumper. And the recess for the locking knob covers him partly.
Temporarily ok but in the long term is not recommended.
So Attention: The iPhone4 Bumper / cases ... do not always fit on the iPhone4s. Problem is the displacement of the locking knob !!!