What is the division of tasks among the control towers at Chicago O'Hare airport?
As mins pointed out, the four towers you are asking about:


Old tower, no direct responsibility
Center tower
South tower
North tower


The airport directory (accessible from AirNav) breaks out the different tower and ground frequencies.


TWR N - 9L/27R
TWR C - All others
TWR S - 10R/28L


This makes sense, as the north and south towers shown on the AD are really only close to those applicable runways. The center tower is located where it can handle the rest. The old tower is not labeled.

The ground control is also split between the towers. I couldn't find a good reference for the division of responsibility there, but I would imagine that it would be similar to that of the towers, only controlling the taxiways associated with those runways, which are hard to see from the center tower.