In the fourth season of ABC's popular medical drama, Meredith (Ellen Pompeo), Cristina (Sandra Oh), Alex (Justin Chambers) and Izzie (Katherine Heigl) graduate to resident status, while George (T.R. Knight) must repeat his intern year after failing his exam. That's not George's only problem: He must also deal with the fallout of betraying his marriage vows to Callie (Sara Ramirez) as he and Izzie try to figure out if they truly are more than friends. Callie lets the stress of her marriage get to her, and it affects her ability to perform as chief resident, a position that Bailey (Chandra Wilson) coveted. Bailey's own marital woes come to a head when her little boy is hurt in an accident at home, while the Chief (James Pickens Jr.) makes strides in repairing his marriage after his ill niece returns to the hospital. Cristina deals with the pain of her failed wedding to Burke by throwing herself into her other love, cardiothoracic surgery. But much to her dismay, Erica Hahn (Brooke Smith) is the new head of this department, and Cristina and Erica clash. Also new on staff is Lexie Grey (Chyler Leigh), George's fellow intern and Meredith's half sister, whom Meredith does her best to avoid. Meredith is on the outs with Derek (Patrick Dempsey), which leaves him free to pursue a nurse named Rose (Lauren Stamile). Meanwhile, a group of nurses whom Mark (Eric Dane) has dated band together to try and stop his promiscuous ways. This prompts Mark to want to change, but he's not the only one looking for self-improvement -- Meredith begins therapy with Dr. Wyatt (Amy Madigan). She then reconnects with Derek on a professional level as they work on clinical trials on patients with brain tumors. Alex also reconnects with someone as former patient and love interest Rebecca (Elizabeth Reaser) reenters his life. However, things with her eventually turn complicated, for reasons he doesn't see at first.