Question: Can it be assumed that in 4th Nephi, the name Lamanite no longer referred to just a descendant of Laman or Lemuel?
Answer: Beginning in 4th Nephi 1:20, the word Lamanite refers to a general class of people– those who reject Jesus Christ, reject His holy prophets, and rebel against His Church. It is not known whether those who now took upon themselves the name of Lamanites were just descendants of Laman or Lemuel. They may have included any unrighteous in the land.
“...and there was still peace in the land, save it were a small part of the people who had revolted from the church and taken upon them the name of Lamanites; therefore there began to be Lamanites again in the land” (4 Nephi 1:20).
“And it came to pass in this year, yea, in the two hundred and thirty and first year, there was a great division among the people.
“And it came to pass that in this year there arose a people who were called the Nephites, and they were true believers in Christ; and among them there were those who were called by the Lamanites–Jacobites, and Josephites, and Zoramites;
“And it came to pass that they who rejected the gospel were called Lamanites, and Lemuelites, and Ishmaelites; and they did not dwindle in unbelief, but they did wilfully rebel against the gospel of Christ...” (4 Nephi 1:35-36, 38).
Source: 400 Questions and Answers About the Book of Mormon by Susan Easton Black, p. 190.