The Grey's Anatomy Season 4 Cast Includes: Ellen Pompeo as Meredith Grey, Patrick Dempsey as Derek Shepherd, Sandra Oh as Cristina Yang, Katherine Heigl as Isobel "Izzie" Stevens, T.R. Knight as George O'Malley, Justin Chambers as Alex Karev, Chandra Wilson as Miranda Bailey, James Pickens, Jr. as Richard Webber, Sara Ramirez as Callie Torres, Eric Dane as Mark Sloan and Chyler Leigh as Lexie Grey.