Courtesy of iFixit. I have also verified this issue with the CDMA Titanium back that is sold by Mas Design. The picture below shows stock CDMA back on a 4S and the original 4S back. It is close, but it is enough to obscure a good part of the camera and allow dust in on the top because it does not provide a good seal.
Source: HERE iFixit
Source: HERE iFixit