Mormon's purpose in abridging the plates is not to give us a long description of every war which occurred among the Nephites. Chapter 4 chronicles a war which was about as extensive as the ones described in Alma 43 to Alma 62. In fact, never had the Lamanites had such military success, capturing the land of Zarahemla and also all the lands in the land southward. However, Mormon does not bother us with the details. He doesn't give us warfare for the sake of warfare. He never elaborates on the strategic successes of the Lamanites. As a Nephite general, it would have been too painful for him to write them.
Rather, he described the warfare in the end of Alma because it taught us certain lessons. It provided great examples of integrity and strength in Moroni, Pahoran, Helaman, and his stripling warriors. Having given us such a characteristic description of Nephite and Lamanite warfare, he abridges the current war into less than a chapter because he has already taught us the major lessons to be learned.