A Fourth Book of Occult Philosophy appeared in print in 1559, twenty-four years after Agrippa died, presenting itself as the practical completion of the three genuine books: the operative material, with the spirits, the seals and the actual procedure for conjuration.
It is spurious, and this was established immediately by the person best placed to know. Johann Weyer had been Agrippa's own student and lived in his household, and in De praestigiis daemonum (1563) he denounced the Fourth Book as a forgery falsely fathered on his master. That verdict has been challenged only occasionally since, Stephen Skinner has argued for some authentic material in it, and Thomas Willard has proposed Jacques Gohory as the actual author, and it stands.
This matters practically rather than pedantically. Later grimoire compilations, the Heptameron attributed to Peter of Abano and much of what descends into the Golden Dawn and modern ceremonial practice, draw on the pseudo-Agrippa alongside the real one, and the two are routinely cited together as though a single author had written both. A good deal of what is confidently attributed to Agrippa in occult writing comes from a book he did not write, published after his death, by someone using his name because it sold.