The fourth volume is smaller than the others, and Christopher Tolkien describes how it was part of a larger volume called Sauron Defeated, and was separated out. Apparently, the original ninth volume of the twelve volume series was titled Sauron Defeated and included two additional sections; Part Two: The Notion Club Papers*, and Part Three: The Drowning of Anadûnê. ** Because parts one and two weren't specific to the history of History of The Lord of the Rings, Part One was separated out in the paperback series I purchased, published by Houghton Mifflin.
The four book series traces Tolkien's original writing process as the story developed over the decades, and as he did in the previous three volumes, Christopher Tolkien does his best to establish the chronology on the various drafts of the closing parts of the saga based on the completeness of the drafts, how close they are to the final story, where the corrections were, and how the process worked for his father's writing process.
The End of the Third Age begins with Sam and Frodo in Mordor, and ends with the story's epilogue. Christopher Tolkien says in the foreword that he has not attempted to trace the history of the appendixes of The Lord of the Rings in this volume given the challenges that task would entail.
If you're a fan, and/or you're interested in how The Lord of the Rings was written, or perhaps a writer, and interested in the writing methods of Tolkien, the this series is worth a read.
* The Notion Club Papers is an unpublished story by Tolkien of a fictionalized version of his own club, the Inklings, in which members time travel in spirit to the past to witness 'mythological' stories, such as the ruin of Numenor.
** This is a history of Tolkien's writings on the fall of Numenor.
