It depends on the waveform tool you're using.

If it's wavescan or ViVA, you'd plot the 4 bits, then select the 4 bits in the graph, and do Trace->Bus->Create (or use the create bus icon). Make sure the LSB/MSB order is correct, and create the bus.

Then select the resulting bus waveform, and hit the d2a icon. Make sure that "Busses output as" is set to "Analog Voltage". Set the Analog Hi Voltage to 15 (i.e. 2^NBITS-1), and the low voltage to 0. Hit OK, and you'll get an "analog" waveform with a value 8.

If using simVision, you can select the bus, then do Format->Trace->Analog Sample/Hold .

Regards,

Andrew.