CHARMED - The official word from the WB
For three spellbinding years, the Charmed Ones - Prue, Piper, and Phoebe - have used their witchcraft and the "Book of Shadows" to keep themselves alive as they save innocent souls and fight the darkest evil. Now, this season, the unthinkable will happen. The power of three will be broken forever.
So begins a new chapter in the CHARMED saga as the surviving sistersí search for a way to stay alive and strengthen their fragile bond while they continue to vanquish the diabolical. The sisters find renewed faith in the form of an unexpected revelation: there may be yet another Halliwell offspring they never knew about - one who could reunite the power of three and return these virtuous witches to their former glory. They'll find that fighting the good fight is a deadly occupation, but one they were born to do.
From executive producers Aaron Spelling, E. Duke Vincent, Connie Burge and Brad Kern, with Spelling Entertainment.

Side Note: In the list of actors on the fourth season of Charmed Alyssa is actually listed at the top and Julian is still included as part of the cast. Shannen is obviously gone.