The authors present a simulation model and results for a four user OCDMA system operating at an overall data rate of 4 Gb/s (4 x 1 Gb/s). The work presents the novel use of a nonlinear detector based on TPA for simultaneous thresholding and detection in an OCDMA system. The detection of the decoded optical signal is also performed using a linear (PIN) detector separately. It is demonstrated that the use of the nonlinear detector can improve the power penally associated with the transition from a single user system to a four user system by approximately 8.5 dB in comparison to the same system using a linear detector. These results are in good agreement with experimental results already taken.