Take a look at the picture from Apple, showing both the Black and the White 4s models. Do you notice that the White has what looks to be two ear piece slits and the Black only has one?
The proximity sensor works off infrared technology (like your average TV remote) it requires a very dark cover over the sensor. You cannot hide it on a white phone, so your thread title is incorrect. The only difference is in the colors. The proximity sensor is the same in either model.