Season four opens with the live episode "Ambush", performed twice (once for the East coast, once for the West coast). Dr. Morgenstern, head of the ER, has a heart attack, threatening his life. Weaver agrees to temporarily step in until he recovers. Unfortunately, when Morgenstern does come back, he cannot perform surgery like he used to do because he now knows what it is like to be a patient. After a surgical accident takes a man's life, Morgenstern decides to permanently step down as head of the ER and he leaves the hospital.