The Last Hope (Warriors: Omen of the Stars #6) by Erin Hunter

The end of the stars draws near. Three must become four to battle the darkness that lasts forever...

After countless moons of treachery, Tigerstar's Dark Forest apprentices are ready to lay siege upon the warrior Clans. As the Clan cats seek out their allies and enemies, Jayfeather, Lionblaze & Dovewing search desperately for the fourth cat who is prophesied to lead the Clans to victory - and who may be their only hope for survival.