Charlize Theron is rumored to be slated to star as the female lead in Mad Max 4, according to E! Online.  The film, which will be directed by George Miller (who made the first three films in the series), has been in development for several years but Miller is apparently now auditioning cast members, according to the report.  

Miller has said publicly that he thinks Mel Gibson is both too old and probably uninterested in doing a fourth film, and is reportedly looking at Tom Hardy as the male lead.