<Li> Sterling Knight as Kip </Li> <P> The fourth season had twelve roles receiving star billing, with ten of them returning from the previous season, eight of whom are part of the original cast from the first one . All the actors who are billed as series regulars portray physicians from the surgical wing of the fictional Seattle Grace Hospital . The majority of the show's episodes are narrated by Ellen Pompeo, who portrayed protagonist Dr. Meredith Grey, a surgical resident whose storylines are the series' focal points . Sandra Oh acted as Meredith's best friend, highly competitive resident Dr. Cristina Yang . Fellow resident Dr. Isobel "Izzie" Stevens was portrayed by Katherine Heigl, while Dr. Alexander "Alex" Karev was played by Justin Chambers . T.R. Knight acted as insecure resident with self - confidence issues, Dr. George O'Malley, whereas Chandra Wilson portrayed Chief Resident and general surgeon Dr. Miranda Bailey, former mentor of the five residents during their internship . James Pickens, Jr. portrayed attending physician and general surgeon Dr. Richard Webber, who continues his position as Chief of Surgery, despite his former wishes of retirement . Orthopedic surgeon and fifth - year resident Dr. Calliope "Callie" Torres, who was portrayed by Sara Ramirez, has to face her husband's unfaithfulness and her unexpected bisexuality . Attending plastic surgeon, Dr. Mark Sloan was portrayed by Eric Dane, who is constantly seeking reconciliation with former best friend, attending physician and Chief of Neurosurgery Dr. Derek Shepherd (Patrick Dempsey), whose lasting relationship with Meredith Grey faces difficulties . Former Reunion star Chyler Leigh was promoted to series regular status, after short appearances in the final two episodes of the third season, portraying Meredith's half - sister Lexie Grey, who opts for a surgical internship at Seattle Grace Hospital against Massachusetts General Hospital, after her mother's sudden death . Silence of the Lambs star, Brooke Smith was upgraded to series regular status after multiple guest appearances in the second and third seasons . An antagonizing character at first, she replaces Preston Burke as the Chief of Cardiothoracic Surgery, constantly displaying disrespect for Cristina's previous relationship with him . </P> <P> Numerous supporting characters have been given recurring appearances in the progressive storyline, including former Gilmore Girls actor Edward Herrmann who appeared in three episodes . Seth Green of Buffy the Vampire Slayer guest starred in two episodes, whereas Lauren Stamile portrayed nurse Rose, a love interest for Derek . Former regular Kate Walsh appeared for the first time since her departure on May 1, 2008 receiving a special guest star billing in the role of Addison Montgomery, now the main character of the spin - off Private Practice . Jeff Perry, Loretta Devine and Debra Monk reprised their roles as Thatcher Grey, Adele Webber and Louise O'Malley, respectively . Diahann Carroll and Elizabeth Reaser continued their season three - introduced roles as Jane Burke and Rebecca Pope, respectively . </P> <P> In October 2006, news reports surfaced that Washington had insulted co-star T.R. Knight with a homophobic slur during an argument with Patrick Dempsey . Shortly after the details of the argument became public, Knight publicly disclosed that he was gay . The situation seemed somewhat resolved when Washington issued a statement, apologizing for his "unfortunate use of words during the recent incident on - set ." The controversy later resurfaced when the cast appeared at the Golden Globes in January 2007 . While being interviewed on the red carpet prior to the awards, Washington joked, "I love gay . I wanted to be gay . Please let me be gay ." After the show won Best Drama, Washington, in response to press queries as to any conflicts backstage, said, "I never called T.R. a faggot ." However, in an interview with Ellen DeGeneres on The Ellen DeGeneres Show, Knight said that "everybody heard him ." </P>

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