It is shown in the specifications table. It looks like the Plus4 Pro has higher sweep rates (600 pts/sec) vs 200-400 on the Plus4, as well as a 10dB lower noise floor for the same amount of averaging. It seems that 6 dB comes from a tighter measurement bandwidth on the Plus4 Pro. Oddly enough, the accuracy is not stated in the spec table for any of the models. If they are the same (are they?), then it looks like you can save $100 if you can live with waiting longer to get a measurement result.