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.
The Face of the Enemy - S4-E17
Question: What is the song playing in the bar, after Sheridan has been drugged and is fighting off the attackers (when the scene goes into slow motion)?
Chosen answer: It was a piece composed expressly for the episode by Christopher Franke, who simply called it "Bar Background Music."
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.