The Sign of the Four is a detective novel. Sherlock Holmes is one of the defining characters of the detective genre, and this is an early and definitive example of Holmes's trademark style of deductive reasoning.
Doyle's novel is also an example of gothic fiction. A genre that rose to the fore during the Victorian period in which Doyle began publishing stories about Holmes, gothic fiction harnesses the power of the uncanny to tantalize the reader with narratives that exploit mystery, suspense, and dark subject matters. Even Holmes's battle with drug addiction, which Doyle explores in the opening page of his novel, is cast in distinctly gothic intensity:
With his long, white, nervous fingers he adjusted the delicate needle and rolled back his left shirtcuff. For some little time his eyes rested thoughtfully upon the sinewy forearm and wrist, all dotted and scarred with innumerable puncture-marks. Finally, he thrust the sharp point home, pressed down the tiny piston, and sank back into the velvet-lined armchair with a long sigh of satisfaction.
Doyle focuses his description of the sinister effects of this addiction on its physical impact on Holmes's body—after all, the gothic genre predicted many of the tropes of medical and body horror that grew in popularity in the 20th century.
Similar conventions are on constant display throughout The Sign of the Four—from the descriptions of Victorian London to the horrifying portrayal of the aftermath of Bartholomew Sholto's murder, Doyle suffuses his novel with gothic gloom.
