Considering the agreement of 350 A.S. in which the Nephite lands of inheritance were divided, the words here in Mormon 4:15, "they (the Nephites) did beat again the Lamanites, and drive them out of their lands" probably mean that the Lamanites were only forced to retreat southward past the narrow passage (the dividing line in the 350 A.S. treaty). Southward of the narrow passage was presumably the former Nephite land of Bountiful (Alma 22:31-33). Thus, it is not likely that the Nephites again regained all of the general land of Zarahemla as some might presume. [Alan C. Miner, Personal Notes]
Geographical [Theory Map]: Mormon 4:15 The Lamanites Are Driven Out of Nephite Lands (367 A.S.)