榮獲金球獎最佳戲劇類電視劇集！榮獲艾美獎最佳戲劇類影集選角！螢幕演員公會最佳戲劇類電視劇集！故事延續第三季精采的劇情。西雅圖恩典病院因為這些實習大夫之間的愛恨情仇，變得更加熱鬧更有可看性！第三季中格蕾終於擺脫了德瑞克和獸醫間的三角戀情，依自己的心意選擇了所愛的人。伊茲在丹尼死后一厥不振，甚至想要放棄成為外科大夫的夢想。克莉絲汀知道了柏克的秘密后，一向詭計幫他掩飾。而喬治和性感火辣的住院醫師之間的關係變得更迷離。而艾立克斯竟發明自己還是愛著伊茲！另外，馬克為了艾蒂森竟也來到了西雅圖恩典病院工作。Meredith和Cristina蜜月歸來（買好的機票欠好浪費~），Burke已經消失無蹤。客歲的實習大夫Meredith, Cristina, Izzie和Alex，已經獲得了正式駐院大夫的資格，并且有了自己的跟班，一群初出茅廬，青澀而稚嫩的實習大夫。當中，有客歲考試不合格的George，還有Meredith素未謀面的妹妹Lexie Grey。今天，他們將要處理一宗三車連環相撞的交通意外，大量的傷者讓新來的實習七手八腳，木雞之呆。另一邊廂，Derek和Meredith的感情陷入僵局，有可能他們都會和新來的同事有新的感情成長。Bailey對于Callie成為駐院總大夫感到氣憤，也許她一時三刻還難以接受自己現在要向Callie匯報工作。Richard留任外科主任，仍然是大家的老大~(簡介轉自YDY)After failing his exam, George reluctantly faces his first day reliving his internship among a sea of fresh faces. Meanwhile, Cristina, Meredith, Izzie and Alex are in charge of their own interns and George is assigned to Meredith.Neither Cristina or Meredith have heard from her exes since the wedding, and they don't seem overly concerned, which astounds Izzie, who is counting every day since she last spoke to George. George asks to talk to Meredith but before he can get her alone, the rest of the new interns show up in the hall and are shooed off by the residents. George gives up and joins the interns. Cristina nicknames him "Bambi," saying he needs to learn how to deal with his new situation.