Audio problem: In the last scene of the episode Sheridan says to Mr. Sebastian "The morning after the last of a string of murders in the East End." His lips however, say West End instead of East End. The script and initial broadcast used the words East End, but after it was pointed out that this was factually incorrect, the line was dubbed over for all subsequent broadcasts and releases. (00:38:50)
Revealing mistake: When he's attacked in Down Below, Dr. Franklin is shoved against a station bulkhead/wall that wobbles when he hits it. (00:15:30)
Delenn: The third principle of sentient life is the capacity for self-sacrifice, the conscious ability to override evolution and self-preservation for a cause, a friend, a loved one.
Lines of Communication - S4-E11
Trivia: Marcus tells the Mars resistance fighters that he was raised on the planet Arisia. J. Michael Straczynski used that name as an homage to one of his favorite science fiction book series, E.E. "Doc" Smith's Lensmen novels, in which the Arisians were the good guys. (00:16:00)
Question: What did Susan do after she left at the end of season 4?
Answer: She was promoted to Captain and headed a new class of ship called the Warlock. Hence she was no longer on the station after season 4, dealing with her grief from Marcus dying.
Chosen answer: She was promoted to Captain and thus had to leave Babylon 5, becoming a captain on a new Warlock class ship. Later on she retires/quits and becomes the top Ranger on Minbar.