The model / device has 4 BWS inputs, since it is a x36 device.
The test bench has been written to accept 2 BWS inputs, and it copies to the other two. It is a test bench, so has been optimized for execution.
The test vector file can be updated by any user, to include two more bits and the test bench can be modified to accept a 4 bit string, instead of 2 bits. So you can go ahead and change the values in the test vector file as per your requirements.