Elton John threw a 49th birthday party for his husband, David Furnish, at The Four Seasons restaurant Tuesday night. About 60 friends, including B arbara WaltersIngrid Sischy and Sandy Brant, City Council Speaker Christine QuinnTamara MellonBruce WeberRufus WainwrightDavid GeffenCalvin Klein and Billie Jean King gathered in a private room, where sources said Furnish gushed to guests about the birth of their son, Zachary. Furnish was given three cakes from Magnolia Bakery with the words “Happy,” “Birthday” and “David” written on each, and the couple even asked if a piece could be taken out to their driver. [Page Six]