Watson takes Miss Morstan back to her home, feeling that the Agra treasure is “like an impassable barrier” between them. By now, it is nearly 2 A.M. Watson meets Mrs. Forrester, who employs Miss Morstan as a governess; he is pleased to see the warmth between the two women, and the tranquility of their home “in the midst of the wild, dark business which had absorbed us.”
Watson heads for Pinchin Lane to pick up Toby the hound. He finds the house of Mr. Sherman, one of Holmes’ associates, who is glad to lend Toby for service to Sherlock Holmes. Watson returns to Pondicherry Lodge with Toby.
Toby represents a kind of extension of Holmes’ powers, as do the Baker Street Irregulars later on. Holmes has a network of people he can call upon for favors.
Holmes reexamines the small “child-like” footprints of the accomplice, noticing that the toes are “distinctly divided.” He then instructs Watson to go downstairs and let Toby the dog loose.
Doyle builds the sense of the second culprit as an “other,” fitting in with the gothic overlap (like Dracula or Frankenstein’s monster)).
Watson, now outside, observes Holmes clambering on the roof, looking for the accomplice’s method of entrance. Holmes notices a barrel on the ground, and clambers down a drain pipe before lowering himself on to the barrel. He figures that this must have been the means of entry. Holmes shows Watson a small case containing more of the sharp poisoned thorns that he found on the roof.
Holmes thinks that, having found the case of blow darts, the owner of them will no longer be able to employ he deadly weapon.
Holmes then gives Toby a sniff of a handkerchief coated in the creosote from upstairs, hoping this will help them find the criminals. Watson and Holmes follow the hound on a lengthy walk into and around London.
The walk around London gives the story a moment of pause and also allows Holmes to do some thinking.
As they follow Toby, Holmes expands on his views about the case, calling it “simplicity itself.” He identifies the wooden-legged man as Jonathan Small, one of the signatories on “the sign of the four” map in Captain Morstan’s possession. He assumes that Small must have been a convict under the guard of Captain Morstan and Major Sholto, which would explain why he is now seeking his share of the treasure—it was him who told them of the treasure’s whereabouts, presumably on the agreement that they would let him have his fair cut.
Of course, Watson, like the reader, has not come to any of the same conclusions that Holmes’ intellect has brought him to. Interestingly, at this stage there is no discussion of the origin of the treasure or who it first belonged to.
Holmes gets Watson to realize that the letter that so frightened Major Sholto was most likely from the man he had wronged, now free to come for his treasure. Holmes reminds Watson that, according to Thaddeus, Sholto once shot at a white man with a wooden leg; and as there was only one “white man’s name” on the “sign of the four” chart, it follows that the man they are looking for is Jonathan Small.
This moment represents something of an error or oversight in the text—Thaddeus never actually said in the earlier exchange that the wooden-legged man his father shot at was white. The mention of race is characteristic of the text generally, in which the color of skin represents a dividing line, almost exclusively painting non-white people in a demeaning and offensive manner.
Holmes continues that Small must have returned to England now to find his treasure. He probably made contact with the staff of the Sholto household, trying to find an insider that might help him. On hearing that Major Sholto was about to die, he went to the dying man’s window, deterred by the presence of the two sons. Since then, reasons Holmes, Small must have kept “a secret watch” on the efforts to find the treasure.
Holmes’ reasoning often depends upon inference, rather than deduction. His explanation is possible and of course turns out to be true, but it’s conceivable that there could be other explanations. His particular explanation builds a sense of the pain and malice that surrounds the ownership and pursuit of the Agra treasure.
Holmes reasons that Small probably didn’t want Bartholomew dead, but that this was the result of his accomplice’s actions. Holmes thinks he has figured out the identity of the accomplice, whom he describes as having acted on “savage instincts.” He promises to tell Watson “soon enough.”
The word “savage” is particularly key here, as Tonga will later be presented as exactly that—an inferior foreigner. It also creates a distinction between Small and Tonga, painting the former as a more honorable figure despite the entire episode originating from his lust for the treasure.
With London rising to meet the next working day, Holmes and Watson are still walking around London, following Toby. Eventually the hound gets confused, before leading them off again with purpose. However, he has merely led them to a nearby store of creosote. Holmes and Watson laugh at the mishap.
This passage provides a moment of light relief to the darkness and mystery of the story and keeps Holmes from reaching a conclusion too quickly.