This is the second of the Sherlock Holmes novels and opens with Sherlock Holmes shooting up cocaine and Watson sitting in his chair deciding whether he should object or not. In fact, the story opens with Holmes shooting up cocaine and closes with Holmes shooting cocaine which book ends a rather complex and intricate plot. In a way the whole story is about Holmes searching for a way to stimulate himself, and in a sense that is his addiction: stimulation.
The story is about a hidden treasure (which in the end remains hidden) and is a lot different, but still similar, to his previous adventure. The story is different because there is a lot more action, which climaxes with a boat chase down the River Thames that concluded with a gun fight. It is also a treasure hunt, though where Holmes is concerned, only the legitimate owner of the treasure has any right to the treasure. I have suggested that he is amoral, but in a way he still has that sense of justice in that he will only recognise the rightful owner of the treasure. Also, even though the only reason he is a consulting detective is his search for stimulation, this is tempered by the fact that in many cases it is guilty parties that he wishes to uncover.
Sign of the Four seems to set the scene for many of the police dramas that have poured out of Hollywood ever since, despite many of them not being mysteries per se. Doyle also sets the scene for many of the mystery novels that have come afterwards, despite them not being a who done it in the traditional sense (I still have to read some Agatha Christie to see how these stories are set out). It is not that we are given some clues and have to put these clues together and work out the crime before the character in the story, but we follow Holmes around and see how he uses deductive reasoning to solve the mystery. We are never given enough information to be able to solve the mystery. Also, Doyle's structure is that the mystery is revealed halfway through the novel, and then the rest of the novel is the back story (and in this case, the chase scene).
This is not the most famous of the Holmes stories though, that honour goes to Hounds of the Baskervilles, but it seems that this one has been turned into a number movies, twelve to be precise (though none of them were directed by Guy Ritchie, but that is probably because he decided to use his own story, even though I feel that he captures the real Sherlock Holmes better than many of the others – despite his Holmes not shooting up cocaine, which surprises me because Ritchie has never shied away from using drug references in his movies, though he does have him play the violin in a way that I do get from the book). Oh, and Watson also meets his wife and gets married.