<P> The fourth season of Charmed, an American supernatural drama television series, began airing on October 4, 2001 on The WB . Airing on Thursdays at 9: 00 pm, the season consisted of 22 episodes and concluded its airing on May 16, 2002 . This season also saw the introduction of Rose McGowan as Paige Matthews--half - sister to Prue, Piper and Phoebe--and a slight alteration of the opening credits, due to the third season departure of Shannen Doherty as Prue . Paramount Home Entertainment released the complete fourth season in a six - disc boxed set on February 28, 2006 . </P> <Ul> <Li> Alyssa Milano as Phoebe Halliwell </Li> <Li> Rose McGowan as Paige Matthews </Li> <Li> Holly Marie Combs as Piper Halliwell </Li> <Li> Brian Krause as Leo Wyatt </Li> <Li> Julian McMahon as Cole Turner </Li> <Li> Dorian Gregory as Darryl Morris </Li> </Ul> <Li> Alyssa Milano as Phoebe Halliwell </Li> <Li> Rose McGowan as Paige Matthews </Li>

What season does piper find out she's pregnant