As indicated in the essay on numbers in the Book of Mormon (Helaman, Part 1: Context, Chapter 4, “The Meaning of Numbers: Counts and Estimates in the Book of Mormon”), these numbers may be symbolic. The contrast between four thousand and two thousand would not only mean inferior numbers, but also inferior quality. Four is the Mesoamerican number of perfection. (See commentary accompanying Mosiah 7:2.) Mormon is indicating his relative disadvantage, one that was far greater than two thousand men.
Despite this disadvantage, Mormon achieves his first victory over the invaders; but they have been driven far to the north, giving up large tracts of land.
Chronology: The 330th year would be A.D. 322.