Many computer simulation models of emergency departments have been developed to aid clinicians and managers to maintain and improve the performance of their departments. A model is presented that can also be used to understand changes in performance that may occur as a result of the 4-hour target regime in the English NHS. The model simulates the performance resulting from normal activity, and the differences between this and actual performance are revealing. The results from two departments are presented to demonstrate this mode of model use. These reveal the extent of special action taken in some emergency departments as patients approach the target time, and also show the true underlying performance of the departments