The South Col is a saddle between Everest and Lhotse. It is about 8,000 meters or 26,250 feet. This is the beginning of the end for human existances. If you spend too long at this altitude, your body will die.
Camp 4 is the last camp on a climb from the Nepal side and is based at the South Col. Climbers arrive in the ealry afternoon, get some rest and food and leave for the summit in the early evening.
A summit climb and return will take from 12 to 18 hours and most climbers will spend the next night at the South Col.