How does Arthur Conan Doyle present Sherlock Holmes throughout The Sign of Four?

In The Sign of the Four, Sherlock Holmes is first presented as a drug user. It is made evident in the very first paragraph that his use of drugs is a long-established habit:

...the sinewy forearm and wrist, all dotted and scarred with innumerable puncture-marks.

Conan Doyle tells us, through the character of John Watson, that Sherlock uses both morphine and cocaine. As first impressions go, this is not a very good one. However, he also tells us that Sherlock is a man with a good brain—one with "great powers," in fact. Why would a man like this use drugs? Because he is easily bored, and needs something to stimulate his mind, either artificially or naturally. And so we move to the next facet of Sherlock Holmes: the fact that he is the only unofficial consulting detective in the world.

"I am the last and highest court of appeal in detection. When Gregson or Lestrade or Athelney Jones are out of their depths—which, by the way, is their normal state—the matter is laid before me. I examine the data, as an expert, and pronounce a specialist's opinion."

This makes him sound almost incredibly arrogant, but Conan Doyle follows this up by having Sherlock tell us:

"I claim no credit in such cases. My name figures in no newspaper. The work itself, the pleasure of finding a field for my peculiar powers, is my highest reward."

This smacks of modesty.
Conan Doyle, throughout The Sign of the Four, continues to turn this Sherlock Holmes character like a cut diamond in the light, showing us the addict, the egotist, the social recluse, the detached reasoner, the collector of small (yet important) facts, the lecturer, the amateur prize-fighter, the master of disguise, and so on, each facet giving us a more complete picture of the detective until, by the end, we see a man who is very different from the average, someone who may seem "positively inhuman" at times, but is nonetheless very human indeed—and brilliantly so. Yet, at the very end, the jewel has completed its rotation, and...

"For me," said Sherlock Holmes, "there still remains the cocaine-bottle." And he stretched his long white hand up for it.

Sherlock Holmes is the iconic detective who is depicted in The Sign of Four and many other works by Arthur Conan Doyle. He is brilliant but socially awkward and eccentric. Some recent writers have suggested that he shows certain characteristics of people on the autistic spectrum. When he gets bored, he uses drugs. He is a skilled violinist as well and an expert marksman who occasionally uses his walls for target practice. 
He is portrayed as a confirmed bachelor who appears to have little interest in romance or sexual relationships with neither men nor women. He can be a deeply loyal and caring friend, despite rarely showing or expressing emotions. He also has a strong sense of justice.
His main feature, though, is his intelligence and knowledge of many minute elements of forensic evidence that make him able to solve crimes, such as his ability to read tracks with great detail. He is also a master of deduction.