Who is Jack Dandy, Jack Nimble, Jack Ketch and Jack Tar from The Graveyard Book and what is their importance?

Jack Dandy, Jack Nimble, Jack Ketch, and Jack Tar are members of the Jack of All Trades. They travel to London to kill Bod when it appears that Jack Frost will fail to do so. When they enter the cemetery, Bod uses all he has learned about haunting and the supernatural world to stop these four men.