The four man cup is really a false statement. It is really a 3 man wall and one chase (the rabbit).
In this scenario, the three people who operate as a wall follow the disc from side to side and create an obstacle for the offence to overcome. Again, it is critically important that the cup remain as a unit. The rabbit, is generally the freshest member of the zone, and can be changed with a member of the wall whenever he feels the need.
The rabbit in general forces towards the sideline with the hope of creating a line trap. The rabbits main purpose is to deny the dump or at least make it as difficult as possible.
Behind the wall, are three more defenders. In general, one is responsible for the long threat, with the other two responsible for hammers and swings. The one on the side of the disc (on the left in this case), must ensure that there are no hammers coming over the wall, and the one on the other side, must ensure that there is no continuation of the flow if the disc gets to the handler on the right.