Is it possible to configure mDNS Repeater to broadcast separately between networks A and B, and between C and D?
Or alternatively, can the traffic be filtered so if all 4 networks are configured, only broadcast traffic from A to B would be allowed and same for B and C.
This binary can be run multiple times to create what you want to achieve.
Until proper plugin, you have to run manual.
https://forum.opnsense.org/index.php?topic=48150.0