Over the rocks, in the crevice of which the candle burned, there was thrust out an evil yellow face, a terrible animal face, all seamed and scored with vile passions. Foul with mire, with a bristling beard, and hung with matted hair, it might well have belonged to one of those old savages who dwelt in the burrows on the hillsides. The light beneath him was reflected in his small, cunning eyes which peered fiercely to right and left through the darkness, like a crafty and savage animal who has heard the steps of the hunters.
Physiognomy has a long and illustrious history, from Chaucer's gummy-toothed travelers up through the early twentieth century. The assumption that physical features match personality and temperament comes through strongly in this quote, where the sinful convict ends up looking like a beady-eyed rat.
Interestingly, Doyle's picture of a debaucherous man who looks the part resonates with another novel of the same period, The Picture of Dorian Gray. Oscar Wilde and Arthur Conan Doyle received commissions from the same publisher on the same night in 1889, Wilde for Dorian and Doyle for The Sign of Four. Doyle's physiognomy is also expressive of a classist sentiment, whereby the uneducated, ignoble criminal looks "like a crafty and savage animal" while the evil nobleman looks just like everybody else.