One of the most unique Easter eggs in Black Ops 4 can be found on the Blood of the Dead map, which is one of the three Zombies maps in the game. This one is a conversation between two characters from Black Ops 2 -- Richtofen and Stuhlinger.
As you may know, Richtofen is one of the playable characters in Blood of the Dead. If you play the map in co-op mode with either you or a buddy playing Richtofen, let the player not playing Richtofen die.
When that player starts spectating, you will hear the voice of Stuhlinger, speaking to you through Aether.
Too bad the voice won't help your buddy blast those zombies into glorious giblets.