Fourth Day

The Fourth Day is the novel's fulcrum, the day on which the abbey's accumulated secrets begin to surface with violent clarity. The discovery of Berengar of Arundel's corpse in the balneary, the deciphering of Venantius of Salvemec's zodiacal code, the arrival of the papal legation under Bernard Gui, the public trial of Remigio of Varagine, and the murder of Severinus of Sankt Wendel all occur within this single, catastrophic span of hours. The day transforms the investigation from a local puzzle into a crisis that implicates the entire community, the Franciscan order, and the imperial-papal conflict, while simultaneously revealing the hidden hand of Jorge of Burgos behind the library's deadly secrets.

Discovery of Berengar's Body and the Poison Trail

The day begins before dawn when William of Baskerville and Adso of Melk, searching the balneary after Alinardo of Grottaferrata's cryptic warning about the third trumpet, find Berengar's naked, drowned body in a tub. Severinus of Sankt Wendel, the herbalist, notes that the corpse's tongue is black and that the fingertips of both Venantius and Berengar are darkened, suggesting they ingested a common poison. This observation, combined with the discovery of a bloodstained cloth under Berengar's pallet, leads William to deduce that the deaths are not random but linked by a toxic substance. Severinus recalls a powerful poison stolen from his laboratory years earlier, a yellowish slime that kills within half an hour, and admits he had discussed it with Malachi of Hildesheim and possibly Berengar. The trail of evidence now points unmistakably to the library and a forbidden book.

Deciphering the Zodiacal Code and the Library's Map

Later that morning, William deciphers Venantius's zodiacal code, revealing the message: "Secretum finis Africae manus supra idolum primum et septimum de quatuor." Though the meaning remains obscure, the breakthrough confirms that Venantius had discovered the key to the forbidden "finis Africae"—the secret room of the library. Simultaneously, William reconstructs the library's floor plan from the outside, deducing that it contains fifty-six rooms arranged around an octagonal well, with four heptagonal towers. He realizes the verses from the Apocalypse that mark each room are not merely decorative but form an alphabet, with the initial letters spelling out the names of regions (HIBERNIA, YSPANIA, LEONES, etc.), mapping the library as a microcosm of the known world. This intellectual triumph, however, is immediately overshadowed by the arrival of the papal legation.

Arrival of Bernard Gui and the Trial of Remigio

The papal legation, led by Cardinal Bertrand del Poggetto and the inquisitor Bernard Gui, arrives at the abbey. Bernard Gui, a figure of cold, methodical cruelty, immediately takes charge of the investigation. That afternoon, he conducts a public trial of Remigio of Varagine, the cellarer, who has been arrested after Salvatore's confession reveals their shared past as followers of the heretical Fra Dolcino. Under Bernard's relentless questioning, Remigio breaks down, confessing not only to his Dolcinian past but also, in a fit of despair and madness, to all the murders in the abbey—a confession William knows to be false. Bernard's triumph is complete: he has linked the Franciscan ideal of poverty to the violent heresy of the Dolcinians, effectively sabotaging the peace conference between the Franciscans and the papal court. The cellarer is condemned to be taken to Avignon for execution.

Murder of Severinus and the Vanishing Book

While the trial is underway, Severinus, who had earlier found a strange book in his laboratory—the very book Venantius had stolen from the library—sends an urgent message to William. Before William can reach him, Severinus is murdered, his skull crushed by an armillary sphere. The book has vanished. William and Adso, searching the laboratory, realize the book was a composite volume containing Arabic, Syriac, and Greek texts, and that they had overlooked it because they were looking only for a Greek manuscript. The book is later recovered by Benno of Uppsala, who, in a bid for power, hands it over to Malachi in exchange for the position of assistant librarian. Malachi, acting on Jorge's orders, returns the book to the finis Africae, but the chain of violence is not broken. The day ends with the abbey in chaos, the conference in ruins, and William on the verge of the final revelation.