My last 4S had that issue, plus the top area was significantly dimmer than the bottom and it had a yellow 'crust' around the top edge of the screen. Bugged me whenever I was reading something on the screen. The screen on my new 4 is much better.
BTW, does your serial number start with C39? 3/3 of phones with a C39 prefix that I have seen have exhibited this issue to a certain extent. The best 4S screens I have seen seem to be DNP or DNQ prefixed.
I've got that yellow crust on the bottom of my screen: it about 1/4" all the way across. When reading a web page landscape that edge looks dirty.