Just as The Grimswell Curse was an homage to The Hound of the Baskervilles, The Devil and the Four is a sort of theme and variation upon The Sign of the Four, and the two titles deliberately echo each other.  Past conspiracies and misdeeds again come back to rebound upon the four perpetrators. 

 

The wine and spirits merchant John Hardy hires Holmes to try to find out why a mysterious note mentioning 'four for the devil' and including an obituary notice for the artist Gaston Lupin, has so badly frightened his wife Marguerite.  She has left for Paris to seek the help of a mysterious detective, and Holmes and Vernier depart to seek her out.  They soon discover what appears to be a missing Botticelli masterpiece stolen some twenty years before.  Its owner the Count de Laval was stabbed to death, and after a sensational trial,  the beautiful young Simone Dujardin was sent to prison.  Marguerite Hardy fears diabolical influences, but Holmes suspects a vengeful person from her past. 

 

Siciliano's depiction of  fin-de-siècle Satanism in Paris and, in particular, of a Black Mass, owes a debt to Joris-Karl Huysman's bizarre 1891 novel Là-Bas (Down There).  The grand cathedral of Saint-Sulpice with its bell towers also plays a major role in both books.

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From the reviews:

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In Siciliano’s superior sixth Sherlock Holmes pastiche (after 2017’s The Moonstone’s Curse), Marguerite Hardy, a Frenchwoman living in London, is badly frightened when someone sends her a newspaper clipping describing the death, apparently from a heart attack, of French artist Gaston Lupin, along with a letter identifying Lupin as the first of “Four for the Devil” and herself as the third. When her husband, John, a wine and spirits dealer, suggests employing Holmes, she forbids it before departing for Paris to seek the aid of a female amateur detective. John ignores his wife’s wishes and consults Holmes, who agrees to carry on a discreet parallel inquiry. The detective—accompanied by Dr. Henry Vernier, his cousin who serves as the series’ Watson stand-in—travels to Paris, where he discovers that the dead man was a talented art forger and comes to suspect that Lupin was given a fatal overdose of morphine. The case takes a major unexpected twist when the identity of the investigator Marguerite turned to is revealed. Although this Holmes is not always the same as Conan Doyle’s, Siciliano makes the divergences plausible under the circumstances.

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                                                Publishers Weekly, August 27, 2018

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