Even Sherlockians disagree at times, and always have.
Did Sherlock Holmes have a romantic relationship with Irene Adler? (No way.) Is “Martha” in “His Last Bow” Mrs. Hudson? (See previous.)
And is the second Holmes novel The Sign of the Four or The Sign of Four? Let’s look at the facts.
The novel first appeared as The Sign of the Four in Lippincott’s Monthly Magazine in February 1890. And that is the way the phrase appears within the text itself. According to Randall Stock's manuscript inventory at www.bestofsherlock.com, "Conan Doyle later shortened the title to simply The Sign of Four." The British and American first book editions used the shorter version, but even a modest-size Sherlockian library inevitably has multiple examples of both titles.
Christopher Morley, founder of the Baker Street Irregulars, preferred The Sign of the Four and used that version of the title in his annotated anthology, Sherlock Holmes and Dr. Watson: A Textbook of Friendship. Edgar W. Smith, his successor, did likewise in what is often considered the BSI edition of the Canon from Heritage Press.
William S. Baring-Gould’s groundbreaking Annotated Sherlock Holmes followed their example, although the estimable Leslie I. Klinger’s New Annotated doesn’t.
Morley, Smith, and Baring-Gould constitute a formidable trio on one side of the controversy, but their street cred is more than balanced out by three gentlemen on the other side: Watson, Holmes, and Conan Doyle.
In the original Strand magazine publication of the stories, Dr. Watson calls his second book The Sign of Four in “A Case of Identity” and “The Five Orange Pips;” Sherlock Holmes does so “The Cardboard Box” and “The Stock-Broker’s Clerk.” Arthur Conan Doyle used the shorter version of the title in Chapter VIII of his Memories and Adventures.
Case closed! It’s The Sign of Four.
But if you disagree, I respect your opinion.