What’s that? A fourth Powerpuff Girl? How can that be? What’s it all about?  Join Blossom, Bubbles and Buttercup when Cartoon Network unveils an all-new, superpowered, fourth Powerpuff Girl during The Powerpuff Girls: Power of Four, premiering Sunday, Sept. 17 at 5:30 p.m. (ET/PT). The Powerpuff Girls: Power of Four is a five-part movie where…