Info on the Zones is found on Pgs. 96-100, and on Pg. 176 of the manual.
Assuming you have everything connected via HDMI: If I understand the chart on P. 176 correctly, main-zone HDMI inputs will be downmixed to stereo for the Zone 4 RCA outputs.
If all else fails, you can get a HDMI to 5.1 analog converter and connect it to the 5100’s HDMI Zone 2 output.
The Zone 2 HDMI out delivers a discrete signal output signal, totally separate from the HDMI ARC output.
In addition to 5.1, the converter has a setting for 2-channel. It will downmix the HDMI’s 5.1 signal to 2 channels that you can connect via RCA cables to your Nakamachi.
However, how listenable the downmixed recordings will be is anyone’s guess. Hopefully it is as good as what the headphone jacks deliver.
Regards,
Wayne A. Pflughaupt