Fourth Nephi really only serves a single purpose and that is to boil things down quickly and deplete the timeline from Christ to the fulfillment of Alma’s 400 year count-down prophecy leading to apostasy and total Nephite doom. And by golly, all of Mormon’s attention is to get that prophecy fulfilled no matter what and do it quickly. The original prophecy was given by Alma:
Alma 45:10,11 wrote:And these are the words: Behold, I perceive that this very people, the Nephites, according to the spirit of revelation which is in me, in four hundred years from the time that Jesus Christ shall manifest himself unto them, shall dwindle in unbelief.
Yea, and then shall they see wars and pestilences, yea, famines and bloodshed, even until the people of Nephi shall become extinct—
Then, some 67 years later, the prophecy is again repeated in a voice of warning by Samuel the Lamanite:
Helaman 12 wrote:5 And he said unto them: Behold, I, Samuel, a Lamanite, do speak the words of the Lord which he doth put into my heart; and behold he hath put it into my heart to say unto this people that the sword of justice hangeth over this people; and four hundred years pass not away save the sword of justice falleth upon this people.
9 And four hundred years shall not pass away before I will cause that they shall be smitten; yea, I will visit them with the sword and with famine and with pestilence.
10 Yea, I will visit them in my fierce anger, and there shall be those of the fourth generation who shall live, of your enemies, to behold your utter destruction; and this shall surely come except ye repent, saith the Lord; and those of the fourth generation shall visit your destruction.
Notice how Alma more or less equates a generation as a value of 100 years and four of them will eventually pass before the end comes. Fourth Nephi specifically names the
“first” (verse 18) and
“second” (verse 22)
generation by the 100 and 200 year marks. Verse 45 alludes to the third generation saying
“And it came to pass that when three hundred years had passed away”.
Moroni gets the privilege of noting the fulfillment of the 400 year prophecy and tells the tale of total doom:
Mormon 8:6 wrote: Behold, four hundred years have passed away since the coming of our Lord and Savior.
Thus we see how Fourth Nephi mainly serves to pass 286 years quickly in order to advance to the 400 year mark given by Mormon. Fourth Nephi is pretty much a quick and fast means to introduce young Mormon into the story and allow him and his son Moroni to fulfill the 400 year prophecy and close the book.
Here is the complete timeline which combines Fourth Nephi with Mormon and Moroni showing the four generations in
red:
34, 35, 36, 37, 38, 39, (40 & 41), 42, 49, (50 & 51), 52, 59, 71, 72, 79,
100, 110, 194,
200, 201, 210, 230, 231, 244, 250, 260,
300, 305, 320, 326, 327, 330, 344, 345, 346, 349, 350, 360, 361, 362, 363, 364, 366, 367, 375, 379, 380, 384,
400, 420