I had a iphone 4 before buying the 4s and tried using the otterbox defender case I used on my 4 and there were two problems, first the camera would take really poor pictures because they re-did the camera in the 4s ( the 4s case has a huge round hole and the 4 a small hole) and the second problem was the switch on the side of the 4s that allows you to turn the ringer on and off was lower on the 4 then the 4s so it was a bit harder to use the switch on the 4s in a otterbox defender 4 case. I bought a 4s case and everything is fine now. I would not use the defender 4 case on your 4s unless the two reasons above don't make a difference to you
best regards