Can Sir Arthur Conan Doyle's The Sign of Four be classed as a hybrid work that mixes genres and writing styles? Why?

Sir Arthur Conan Doyle's The Sign of Four is a detective novel. This is a genre which evolved in English literature out of certain elements of the sensation novel. While The Moonstone (1868) by Wilkie Collins is often considered the first proper detective novel, many other works by sensation novelists like Collins and Mary Elizabeth Braddon start with some sort of mysterious crime and follow a narrative trajectory in which the main plotline involves discovery of the identity of the criminal. Looking further back in history, some have even argued that Sophocles's Oedipus Rex was the first detective story.
In terms of genre, the mystery or detective novel is usually considered to be plot- rather than character-driven. It has fast-paced action, with a mysterious act or crime that occurred before or at the start of the novel being solved by the end of the work. There is usually a complex plot with many twists, red herrings, and clues that only fully make sense to the reader when all is revealed in the final chapters. Characters tend to be painted in fairly broad strokes, often having strikingly distinctive characteristics but lacking complex psychological portraits. Doyle's novel is thus typical of the genre as it evolved in the Victorian era.

Arthur Conan Doyle’s works featuring the brilliant Sherlock Holmes are regarded today as some of the greatest detective fiction ever written. It must be kept in mind, however, that before Conan Doyle, “detective fiction” was not a specific genre. Edgar Allan Poe had made significant progress in popularizing this type of story with his detective Auguste Dupin some sixty years before Sherlock Holmes emerged, and Conan Doyle was far from shy about his admiration for Poe. Conan Doyle did not only write Sherlock Holmes books, however. Decades before, he had begun writing Gothic tales.
In regard to The Sign of Four, therefore, it is appropriate to consider it both a detective story and a Gothic tale. The central case that occupies Holmes—solving the mystery that Mary has brought to him—puts it squarely within the detective fiction genre. Elements that link it to Gothic romance include the character of Mary, an innocent damsel in need of aid; the mysterious settings of London’s dark streets, especially the gloomy, castle-like Pondicherry Lodge; the search for lost treasure; and the exotic, Orientalist elements of the Indian islands.
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