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.