Arthur Conan Doyle, approached by a popular magazine for a contribution, thought up Sherlock Holmes and proposed a story called The Sign of the Six. Posterity knows that the story was numerically diminished, and published as The Sign of the Four.
As signs go, a 6 looks like it's in curly retreat whereas a 4 seems like it's making a protruding statement. It's the dramatic difference between Zorro and his Bangladeshi counterpart ëJorro': in three intersecting strokes of the whip the Latino cowboy establishes his authority and its promise of vengeful justice. Jorro, however, has some difficulty getting the whip to hook into a 'J'.
The word chhakka which is, beyond dispute, the name of the number 6 in 'Indian' (Hindustani), has a more graphic provenance.
But first its definition. It's a noun used exclusively to describe males. There is no such thing as a chhakki. The stigma of the six does not apply to women. It describes the ineffectual and the incompetent, the wretch who is ludicrously unequal to a task.
And there lies the key to the metaphor....