Season four chronicles Clark and his classmates' senior year of high school and centers on his attempt to unite the three stones of knowledge, and trying to cope with Lana's new relationship with Jason Teague. Clark's friendship with Lex becomes increasingly strained, as he begins to distrust Lex more and more. At the end of season 3, Sam Jones III left the series as Pete Ross, and Jensen Ackles was brought in as Jason Teague and given star billing. Erica Durance was cast as Lois Lane, and became a recurring character for 13 episodes. Writers also brought in other popular DC Comics characters, such as Bart Allen, Mister Mxyzptlk, and Sam and Lucy Lane.