Many were killed, and the leaders of the insurrection were imprisoned. Significantly, some version of trial was expected, since Mormon takes pains to explain that imprisonment without a trial was justified by the circumstances, something he would not have done unless he knew his readers expected trials.
“Four thousand” is a suspicious number. Whenever “four” appears in a Mesoamerican context that is not a calendar (and at times when it is), it is important to consider it as a possible symbol, rather than enumeration. “Four” was the number of completion in Mesoamerican cultures, and four thousand deaths could easily mean the “complete” destruction of the dissidents. (See Helaman, Part 1: Context, Chapter 4, “The Meaning of Numbers: Counts and Estimates in the Book of Mormon.”)