Question: In the opening credits, Holly Combs is credited with "and Holly Marie Combs as 'Piper'". Why was she credited with her character's name, when none of the other three lead actresses were?
Answer: She became a producer.
Incorrect. Her billing changed in season 4 but she didn't become a producer until season 5.
Chosen answer: Her agent/manager negotiated a special billing as part of her contract. It distinguishes her from the rest of the cast, probably when she became a producer on the show.
JC Fernandez