<P> The season four premiere "Charmed Again" reveals Leo arrived in time to heal Piper, but Prue died of her injuries, reflecting Doherty's departure from the series . Leo assisted Phoebe to escape the Underworld, absconding on the Source's deal . With Prue dead, the sisters are no longer the Charmed Ones, and Cole does what he can to assist Phoebe and Piper from Shax's continued attacks . Piper and Phoebe regain their full abilities--becoming Charmed Ones once again--when they discover their younger half - sister, Paige Matthews (Rose McGowan), alongside whom they finally destroy Shax . </P> <P> In "Black as Cole", Cole proposes to Phoebe whilst they vanquish a low level demon . Later they find that Sykes has been mimicking Belthazor and in a bid to vanquish him Cole takes on his demonic form but has Phoebe create a potion to kill his demonic half if he cannot change back . Before he can change back, a woman looking to revenge his killing of her love picks up the potion and throws it at him, causing Belthazor to die . In "Lost and Bound", Paige gets Cole a job as a legal aid lawyer . However he can't control his emotions and soon quits but not before the landlord who he was arguing with agrees to put the heating back on . </P> <P> In "Charmed and Dangerous", the Source (Peter Woodward) breaks an aged old agreement and consumes the hollow . The Seer (Debbi Morgan) tricks Cole into consuming the hollow which allows him to assist the Charmed Ones in vanquishing the physical version of the Source . It is revealed that the Seer tricked Cole into absorbing the old Source's power, eventually causing him to become the new Source of Evil in "The Three Faces of Phoebe". An older version of Phoebe has turned bitter because of what he has done and warns her to save herself and states that they never got married but she can't help but think what might have been . Present - day Phoebe asks him if he is hiding anything; he says he is not . </P> <P> The Source takes over Cole completely in "Marry - Go - Round". The Seer warns him that marrying Phoebe would be the worst thing possible as they would have a son, the most powerful magical creature ever, who would be on the side of good, following the Charmed Ones' destiny . She tells him the only way the child will grow up evil is if Cole marries Phoebe in a dark ceremony . As the Source, he puts a spell on the Charmed Ones which makes Phoebe spotty and Piper and Paige over sleep . Paige and Phoebe argue as the latter suspects the former of trying to disrupt the wedding, having already made a mistake with the dress . When Paige tries to make an amends with Phoebe's spots it causes her to become invisible . </P>

When does cole became the source of all evil