The 4 is the name of a set of magical books in the Descendants Universe.
They are described as both primer on magic and to contain the 'source code' of all magic.
As magical books, they contain instructions on casting spells and about various creatures from other worlds. Each book posesses some level of sapience, however and this information seems to be dispensed or obfuscated at the book's whim with entire passages appearing or disappearing at will.
No one book is a single volume. In fact, they can be freely copied or even digitized with all resultant copies retaining just as much power and knowledge as the original.
The actual agenda of the books has been stated by Warpstar to be maintaining and spreading magic in the world. Each book, however, seems to have its own methods and philsophy for pursuing this. It has been stated that each book has a Chosen One, but the purpose and privileges this entails are unknown.
The Book of Reason[]
First appearing in Legacy of One, the Book of Reason was recovered from Morganna by Alloy. It is among the least active of the Books, only hiding and revealing information. It was initially written in archaic French and only visible on the astral plane, Laurel painstakingly scanned and translated it.
The Book of Reason has not yet selected a Chosen One as evidenced in the story. Its opposite is The Book of Passions.
The Book of Passions[]
The Book of Passions was the second book to appear, in Strange Times At Dayspring College. It could only be read by Augustus Roe initially. Augustus however wants nothing to do with it and returned it to the out of print section of Dayspring College's library.
It was then scanned into an ebook and likely destroyed. The ebook was downloaded by the members of the Magi Club 5 once it was put online.
Its Chosen One is Augustus Roe and its opposite it The Book of Reason.
The Book of Madness[]
The Book of Madness has not yet selected a Chosen One as evidenced in the story. Its opposite is The Book of Tranquility.
The Book of Tranquility[]
The Book of Tranquility has yet to appear in the main Descendants series.
The Book of Tranquility has not yet selected a Chosen One as evidenced in the story. Its opposite is The Book of Madness.