When GRRM finished writing A Storm of Swords, he intended it to be the halfway point in the series. The fourth book, A Dance With Dragons, was going to pick up the story five years after the end of A Storm of Swords.
However, flaws in this plan forced GRRM scrap what was known as the 'five-year gap.' The original plan was to publish a fourth book, entitled A Feast For Crows, to cover the five years GRRM originally planned to skip.
However, after even more work, GRRM decided that there was too much information to cover in one book. He decided that the segment would have to be covered by two 'parallel' books that take place over the same time period, but each have different POVs.
Book 4, A Feast For Crows is due out late this year. It will cover the events taking place in the south of Westeros, including King's Landing, Dorne, the riverlands and the Iron Islands.Book 5, A Dance With Dragons, will cover the same time period, only from characters in northern Westeros and on the Great Eastern Continent, including the Wall, the north, the Free Cities and Slaver's Bay.