 Chyler Leigh joined the cast as a main character in the fourth season as Dr. Lexie Grey , Lexie had appeared as a guest star in the final two episodes of the third season . On the selection of Leigh for the role of Lexie , Rhimes said : " Chyler stood out ... It felt like she could be Meredith 's sister , but she had a depth that was very interesting . " Dr. Erica Hahn ( Brooke Smith ) , who first appeared on Grey 's Anatomy in the second season , became a series regular in the fourth season . Shortly after the announcement that Smith would be a regular member of the cast , Entertainment Weekly 's Michael Ausiello , reported that her character , Hahn , would depart from Grey 's Anatomy on November 6 , 2008 . E ! Online 's Kristin Dos Santos asserted that Smith 's dismissal from the show had been forced by the ABC network , as part of an attempt to " de @-@ gay " Grey 's Anatomy , but Rhimes countered these claims , saying that " we did not find that the magic and chemistry with Brooke 's character would sustain in the long run " . 
