The Fourth Warrior isn't a quest in its own right, but a placeholder for three quests that you receive when speaking with Juna about the book to which you found the pages. In speaking with her, you can receive all of the following quests:
You must have the book Snuffing the Fireknight in your inventory to be able to get any of the quests. If the book has been placed in your house, the quests will not be offered. If you do not have the book, Juna is a merchant who sells it if you have already completed the collection for it. It's also likely that you have to have read the book before being offered the quest.