in my opinion:
120 erlangs on 4 A interfaces = 120 erlangs on 4 Ater interface = 120 erlangs on 1 AterMux interface.
1 erlang is always 1 erlang. Erlang is related to the number of calls, not to the throughput of each call.
And 1 call on A interface = 1 call on AterMux interface.