Or you could copy the output on channel 4 to another output in JRiver's DSP and then uncheck or delete it for playback. I do that all the time for channels when doing REW measurements. I just leave REW set to output to the left speaker, but use Mix Channels in JRiver to route the sound to other channels for measurement.
Won't work, because during calibration I bypass Jriver so I cannot remap channels or copy output. I was hoping this can be done in the lynx mixer, but am not sure it can, and I certainly have not figured out how. In any event, I am totally fed up troubleshooting software and want to go back to listening to music at some point (in the near future).