Unfortunately, the only way to make GROUP ring cover to voicemail is to have ALL of the users log out of the group, otherwise the call will ring FOREVER, not overflow to the voicemail.
Perhaps you can use a adjunct, put a Primary Coverage button on the 4 extensions to be able to answer, and cover the adjunct to voicemail. If it is Merlin Messaging, the mailbox that answers will be the same extension number as the adjunct, but you can use Phantom Destination to deposit the incoming message in 9991 or 200.