Toby the dog (Symbol)
Besides Doctor Watson, Sherlock Holmes has another helper. It is a dog: the English bulldog Toby. This sensible animal always helps Sherlock to find criminals, and despite of the fact that it isn’t his own dog, this animal nevertheless remains faithful to Sherlock. Toby is a symbol of devotedness and faithfulness, and evidence that Holmes needs a team to solve cases.
Drugs (Symbol)
Sherlock Holmes has an unhealthy habit. He often uses drugs, but he doesn’t deny that it has fatal consequences. Sherlock uses it only in the absence of interesting crimes. He claims, however, that his mind rebels against idleness. One might think that Sherlock is behaving foolishly, but he is also saving himself from his own mind. Drugs thereby symbolize the avoidance from depression and “death."
Investigation (Motif)
The Sign of the Four has the motif of investigation, along with the rest of the Sherlock Holmes stories. Sherlock Holmes and Doctor Watson work tirelessly to solve mysteries, finding the unexpected but logical explanation behind apparently mystic or otherwise baffling events. Here Sherlock tries to solve the disappearance of Mary’s father, uncovering the secrets of the treasure–and the true feelings of Watson for Mary–along the way.
Riches as an obstacle to love (motif)
Watson is ecstatic when he finds out that the riches are gone. He had been hiding his true feelings and trying to deny the spark between him and Mary because he felt that he wasn't good enough for her and that she would frown upon him. With these riches out of the way, he gains the courage to confess his feelings to her, and discovers that she shares them. Here we see that while wealth can be a wonderful thing, it can also be a hindrance in life.
Many sides to the same coin (motif)
Throughout the story, the tale of the Agra treasure grows increasingly complicated. The reader hears the story of the treasure from several people–first from the remaining Sholto and from clues pieced together by Holmes, and from more pieces that filter in from Mary's father Morstan. Next we hear Jones' theories, and Mr. Smith's two cents, before finally getting the whole entire story from Mr. Small in the last chapter. Here we see that each person tells the same story differently, in the light that best suits them. It shows the malleability of the truth, and how the truth can often be whatever people decide it is, rather than what actually happened.
