The 4th novel in the series is called A Feast for Crows. There was a five-year gap between the release of the third and fourth novel in the series. The planned fourth novel (originally called A Dance with Dragons) was split into two, as it had grown too large. These two novels ran
concurrently, with one focusing on events within the Seven Kingdoms and the other focusing on Essos, the North, and the Wall.
One of the reasons that A Feast for Crows took so long to develop is because George R. R. Martin had to scrap the novel after a year of work. He had originally wanted the fourth book to skip ahead five years. This would allow him to age up the characters and skip over a lot of the training (like Sam at the Citadel and Arya with the Faceless Men).
George R. R. Martin was unable to make this work, since it didn't make sense to skip such a huge period. Was Jon Snow able to command the Night's Watch for five years without trouble? Martin decided to start the novel over from scratch and continued on directly from the ending of A Storm of Swords.