“Women are never to be entirely trusted – not the best of them” (129) (Dramatic Irony)
Here, Holmes implies that men shouldn’t trust women at all. He ridicules women saying that women are cunning and artful creatures, who can make a mess of a job; yet he readily and rightly trusts Mary's story, demonstrating an ironic inconsistency in Holmes' overall views.
“But love is an emotional thing, and whatever is emotional is opposed to that true cold reason which I place above all things. I should never marry myself, lest I bias my judgment” (157) (Situational Irony)
Here Sherlock confirms that he views love as a mental impediment, which is why he wishes to avoid it. This is ironic because we would expect a man of Sherlock's staggering intellect to better understand the human condition and relationships; instead, we see that he is surprisingly stunted, emotionally speaking.
"Beside it is written, in very rough and coarse characters, 'The sign of the four–Jonathan Small, Mahomet Singh, Abdullah Khan, Dost Akbar.' No, I confess that I do not see how this bears upon the matter. Yet it is evidently a document of importance. It has been kept carefully in a pocketbook, for the one side is as clean as the other" (98) (Dramatic Irony)
Initially, everyone dismisses this letter as simply another piece of evidence. Yet, it ultimately becomes the most crucial piece, even lending the words on the page to the title of the story–The Sign of the Four. This is a typical example of dramatic irony in mystery literature: often characters will overlook the importance of a piece of evidence until it later becomes clear how important that evidence is. The game of mystery literature is for the reader to recognize the importance of the evidence before the characters do.
