Pirates of the Caribbean: Legends of the Brethren Court: The Fourth Estate was to be fifth installment in the Pirates of the Caribbean: Legends of the Brethren Court novel series.
As stated in the preview of the Legends of the Brethren Court series seen in the last pages of the novel Bold New Horizons from the Pirates of the Caribbean: Jack Sparrow novel series, the Legends of the Brethren Court series was meant to be composed of six volumes. The fifth novel's title was going to be The Fourth Estate, but for reasons unknown, The Fourth Estate was ultimately cancelled and Day of the Shadow, which was originally meant to be the series' sixth volume, ended up replacing the original planned one.
The story of The Fourth Estate would have showed Captain Jack Sparrow and his crew fighting a mysterious band of misfits from France known as the Fourth Estate, hence the novel's name.