ServerTransportListenAddr is ignored when stated second time for the IPv6 address
Currently is possible to run a dual stacked IPv4 + IPv6 bridge.
ORPort can be set 2 times in torrc, once for IPv4 and once for IPv6 - it will open ports on both address families and listen for connections.
ServerTransportListenAddr will only work for IPv4 and be ignored if set second time in the same torrc for the IPv6 family (no obfs4 port open on the IPv6 address).
Log file does not even mention it, it only confirms the IPv4 one: [notice] Registered server transport 'obfs4' at ':port'