The Sign of the Four (1890), also called The Sign of Four, is the second novel featuring Sherlock Holmes written by Sir Arthur Conan Doyle. Doyle wrote four novels and 56 stories starring the fictional detective.

The story is set in 1888.

The Sign of the Four has a complex plot involving service in East India Company, India, the Indian Rebellion of 1857, a stolen treasure, and a secret pact among four convicts ("the Four" of the title) and two corrupt prison guards. It presents the detective's drug habit and humanizes him in a way that had not been done in the preceding novel, A Study in Scarlet (1887). It also introduces Doctor Watson's future wife, Mary Morstan.

Sir Arthur Conan Doyle described how he was commissioned to write the story over a dinner with Joseph M. Stoddart, managing editor of an American publication Lippincott's Monthly Magazine, at the Langham Hotel in London on 30 August 1889. Stoddart wanted to produce an English version of Lippincott’s with a British editor and British contributors. The dinner was also attended by Oscar Wilde, who eventually contributed The Picture of Dorian Gray to the July 1890 issue. Doyle discussed what he called this "golden evening" in his 1924 autobiography Memories and Adventures.

The novel first appeared in the February 1890 edition of Lippincott's Monthly Magazine as The Sign of the Four; or The Problem of the Sholtos (five-word title), appearing in both London and Philadelphia. The British edition of the magazine originally sold for a shilling, and the American for 25 cents. Surviving copies are now worth several thousand dollars.

Over the following few months in the same year, the novel was then republished in several regional British journals. These re-serialisations gave the title as The Sign of Four. The novel was published in book form in October 1890 by Spencer Blackett, again using the title The Sign of Four. This edition was illustrated by Charles H. M. Kerr. The title of both the British and American editions of this first book edition omitted the second "the" of the original title.

Different editions over the years have varied between the two forms of the title, with most editions favouring the four-word form. The actual text in the novel nearly always uses "the Sign of the Four" (the five-word form) to describe the symbol in the story, although the four-word form is used twice by Jonathan Small in his narrative at the end of the story.

As with the first story, A Study in Scarlet, produced two years previously, The Sign of the Four was not particularly successful to start with. It was the short stories, published from 1891 onwards in Strand Magazine, that made household names of Sherlock Holmes and his creator.

reference : Wikipedia, The Sign of the Four