Fragmentary polyphonic choirbook prepared in Milan for use by the Duomo chapel under the direction of Franchinus Gaffurius. The manuscript, severely damaged in a fire in 1906, is currently preserved in a set of boxes at the Archive and inaccessible to the public. It is variously known in the literature by its olim shelfmark “2266”, as “MilD 4” (Census Catalogue), “il quarto codice di Gaffurio”, or, erroneously, as “Librone 4” (that shelfmark refers to a different item altogether). We use here the name “Librone [4]” in order to highlight its belonging to Gaffurius’s set of manuscripts. Mainly copied by one scribe, with interventions by Gaffurius (see the Inventory). It contains primarily motets (including motet cycles), with some Masses, Mass movements, Magnificats, hymns, and a lauda. The identified composers include Gaffurius, Loyset Compère, Josquin des Prez, Giovanni Spataro, and Gaspar van Weerbeke (see the Catalogue).