When using the calling group, the message waiting light comes on when either or both mailboxes have messages, but if both have messages, and you retreive them from ONE of the boxes, the light will go out.
With Merlin Messaging, there are 4 Auto Attendant, with 4 General Mailboxes, which can have different owners. The Merlin Messaging controls the light instead of the Magix, so the light will stay on if the other box still has messages.
If you have a late enough release of Messaging, you also have Phantom Destination, where the outgoing message of a mailbox plays, but the incoming message goes into a different mailbox.
All this to say: Keep 3922 on an adjunct, have it cover to voice mail, have mailbox 9994 as the phantom destination of 3922, and make 3920 the owner of the General Mailbox for Auto Attendant No. 4.
Her personal DID will remain 3922, she will record her outgoing message in mailbox 3922, but check for incoming messages in 9994. The light on 3920 will indicate messages for both mailboxes, and will go out when both mailboxes have been cleared.