When referring to form in The Sign of the Four you can refer to:
Arthur Conan Doyle did not invent the detective story, but built on conventions already established by Edgar Allan Poe in his story The Murders in the Rue Morgue. Conan Doyle said that 'the problem and its solution must form the theme' of a detective story.
Conventions of this form include:
The Sign of the Four begins with a young woman, Mary Morstan, coming to ask Sherlock Holmes for his help locating her father who had mysteriously disappeared some years previously, and to solve the mystery of some pearls which she had been receiving annually in the post. The plot deepens when Holmes, Watson and Miss Morstan are taken to Thaddeus Sholto's house only to discover that his twin brother, Bartholomew, has been murdered.
Conan Doyle presents Sherlock Holmes as a rational and highly intelligent character, while his companion, Watson, is admiring but often baffled by Holmes’ intellect. The book begins with the unexplained disappearance of Mary Morstan’s father and the riddle of the unexplained pearls which arrive in the post, leading the reader to question whether these are clues. The murder is then introduced and the bumbling official detective, Mr Athelney Jones, sets off on the wrong track. The tension of the plot mounts as Holmes and Watson investigate and finally solve the mysteries of who killed Bartholomew Sholto, who sent Miss Morstan the pearls, and what happened to Mary Morstan’s father.