My experience is that the volume buttons are in the same place and the mute switch is a tiny bit lower.
I've made a regular 4 bumper fit my 4S simply by trimming off a tiny bit of the plastic at the bottom of the opening for the mute switch with a sharp knife.
It really was a tiny modification, only about 2mm x 5mm of trimming, and only to the inside, much thinner bit of the bumper case.
Hey guys it turns out the volume buttons are in the same place as on the iPhone 4 but the mute switch is lowered.
My case happens to just fit!
If the mute toggle was an lower it wouldn't have fit but I am able to access the switch without interference. Thanks!