The two armies gathered recruits for the final battle "for the space of four years" (Ether 15:14). This tradition may have been followed a millennium later, when the Nephites gathered to defend themselves against the "Lamanites and robbers" (see Mormon 5:6; 6:2,5).
Warren and Palmer note that this tradition was apparently also followed in Mesoamerica, where it is found in the writings of Ixtlilxochitl, long after Book of Mormon times. When two opposing Mesoamerican armies planned to have a large battle, they gave each other four years to assemble the opposing armies (Chavero, 1952:52). [Bruce W. Warren and David A. Palmer, The Jaredite Saga, p. 9-5]
Geographical [Theory Map]: Ether 15:15 - 15:34 Ether Finishes His Record (Chronology)