The Emptiness of Wealth
One of the strongest themes of Sign of the Four is the shallowness of wealth, and the destruction that can come through it. This theme arises again and again in Sign of the Four, as the Agra treasure directly and adversely affects almost everyone. In the course of the story, the Sheik’s emissary and Bartholomew are both murdered for the treasure, Tonga is killed while fleeing with it, and Small is sentenced to life imprisonment. Additionally, both Thaddeus and his father spent their lives constantly paranoid about wooden legged men, and about strangers in general. The Agra treasure even provides a “romantic conflict” for Dr. Watson, who feels that he cannot marry Miss Morstan for fear that he will appear to be after her money.
"Invisible Evil"
Doyle, like many other mystery writers, makes use of a writing technique that can be thought of as “invisible evil”: the phenomenon of criminal acts committed by unknown/unseen agents. Doyle plays on the fear of the unknown heavily in the course of the story; indeed, the “sign of the four” itself carries with it mysterious and odious connotations. Also, the fact that Bartholomew was murdered in a locked room serves to heighten the tension surrounding the idea of the “invisible assassins.”
Returning
The theme of “returning," either literally or figuratively, is prevalent in Sign of the Four. The purpose of this theme is to heighten the importance surrounding the Agra treasure. Throughout the story, the appearance of the treasure leads to a direct and often tragic change in the lives of the characters. Because of this, it is important that the removal of the treasure would cause the characters to return to their previous position. In the case of Small, a convict, the re-emergence of the treasure leads him down a path that ends in murder; with the removal of the treasure, he is a prisoner once again. Mary Morstan is a charming young woman whom Watson contemplates marrying. With the prospect of Mary becoming an heiress, however, this possibility is removed. When it is discovered that the Agra treasure is gone, Mary returns to a position in which Watson can comfortably propose marriage. Perhaps the most poignant “return” is that of Holmes himself. At the beginning of the story, he can be found taking cocaine. This vice fades away while the problem of the Agra treasure is prevalent, but, as soon as it is solved, Holmes digresses once again.
Appearances Can Be Deceiving
At first, the letter with the sign of the four is thought to be of no importance by Mary, and she brings it along to give to Sherlock almost on a whim. The letter appears to have little to no significance until it becomes the key to solving the case. A similar occurrence takes place when the trio enters the house of the letter writer. From the outside, it is "sorry"-looking and dilapidated, but inside it is a marvelous cornucopia of luxury and expensive items such as Oriental vases, tiger-skins, and a soft carpet (100).
Love Conquers All
Though Watson initially has reservations about his love for Mary, he eventually confesses it to her, overcoming his fear after the Agra treasure is revealed to be lost. His love positively cannot be contained as it "ejaculated from my very heart" (143). One cannot help but wonder whether the author was not also having a bit of a laugh with this double entendre.
Holmes is a Confirmed Bachelor
Upon hearing that Watson and Mary are engaged, Holmes reveals that he feared as much from the way that they were interacting. Though the reader, by now familiar with Holmes, has come to understand his abrasive character, it is now confirmed that Holmes is a misanthrope who does not seek marriage or particular companionship that much. Instead, he is content with his cocaine and his detective work (158).
The Police Get All the Credit
Throughout the Holmes stories, the police at Scotland Yard continually receive the credit for the cases that Holmes solves, thus allowing his to continue his work in relative secrecy. In fact, in this story we see that Holmes' profile has become rather too large, as he has to disguise himself as an old man for fear that the criminal class will recognize him too easily–thereby also playing a prank on Watson and Jones. So, it is to Holmes' benefit when he gives Jones the credit for the case in the end, as it is not the glory that Holmes seeks, but rather the adventure and the puzzles of the case.
