Katie Cassidy is staying within the CW family.  Cassidy, who previously appeared in Supernatural and last season's Melrose Place (which lasted one season before being canceled), will be joining season 4 of Gossip Girl in the Fall for a recurring role.  Cassidy is scheduled for a multi-episode character arc and EW has a few more details about her character:

She’ll play a student at Columbia and a love interest for fellow undergrad Nate (Chace Crawford).

This next bit probably goes without saying, but a Gossip source tells me that Cassidy’s character “will make trouble” for several Upper East Siders, adding, “She is not what she appears.”

Cassidy will first air in the season 4 premiere.

This is great news.  While I wasn't fond of Cassidy's character in season 3 of Supernatural, I grew to love her in Melrose Place (yes, I watched that show...and I am ashamed of this fact).  I think she was the best thing about that show and if she's going to play a bitchy character like she did in Melrose Place, then we're in for a treat.

You can also read an article we previously posted with Gossip Girl season 4 spoilers, including details about the Paris storyline that will open the fourth season of the show.