Short cut: The sign on work is opposite the sign of the change in volume and the direction of change in pressure indicates the direction of change in temperature.
Full explanation:
With respect to the gas you have to use the equation: W = - P(DeltaV)
1 to 2 Gas expands (volume is positive) and thus the work is negative. Conceptually, the gas does work on the system when it expands (ex: pushes the piston) and now the gas has gone down it's work capacity (it has lost some capacity to do work). The gas transfers kinetic energy from its molecules to move the piston and thus the molecules lose kinetic energy (basically losing temperature).
3 to 4 Gas compresses (volume is negative) and thus work is positive. Work is done on the gas by the piston as the piston compresses the gas and now the gas gains more work capacity (it now has the capacity to do more work). The gas gains internal energy and the molecules gain kinetic energy (basically increase in temperature).
2 to 3 Change in volume is 0 thus work is 0. The gas is at constant volume, however, pressure has decreased. Pressure for arguments sake is the rate and force that the gas molecules are colliding with the walls of their container. Since the molecules are hitting the walls with less pressure it's obvious that they've lost kinetic energy and thus there's a decrease in temperature.
4 to 1 Change in volume is 0 thus work is 0. The gas is at constant volume, however, the pressure has increased. The molecules start hitting the walls with greater force and now have greater kinetic energy, thus there's an increase in temperature.
