4 And the things which are upon these plates pleasing me, because of the prophecies of the coming of Christ; and my fathers knowing that many of them have been fulfilled; yea, and I also know that as many things as have been prophesied concerning us down to this day have been fulfilled, and as many as go beyond this day must surely come to pass—
5 Wherefore, I chose these things, to finish my record upon them, which remainder of my record I shall take from the plates of Nephi; and I cannot write the hundredth part of the things of my people.
6 But behold, I shall take these plates, which contain these prophesyings and revelations, and put them with the remainder of my record, for they are choice unto me; and I know they will be choice unto my brethren.
Quick note: I think Mormon is saying that just as the small plates have already been filled with prophecies of Christ, he is choosing the same kinds of things, which he’ll get off the large plates, to include in his “Words of Mormon” addition. That is, he’s adding his own book after Omni, and in that book he will summarize the entire history of the Nephite people, focusing just on the prophecies of Christ and their fulfillment. That is, he’ll do what Nephi did: have one record for a general story and government record, punctuated with important speeches or important conversions, and one record for an archive of visions, prophecies, and so forth. (That record of course will need to include some history, just like Nephi and the prophets did.) But to me, I think this idea is clear: Mormon is preparing an addition to the small plates that is in sync with what Nephi established for the small plates. (Jacob 1:2: “And he gave me, Jacob, a commandment that I should write upon these plates a few of the things which I considered to be most precious; that I should not touch, save it were lightly, concerning the history of this people which are called the people of Nephi.”)
I think in verses 5-6, Mormon is sort of apologizing for the fact that he’s about to leave out a whole lot of history. So he says, “Don’t worry, it’s okay, I’ll put this spiritual record with my abridgment so you’ll have some more of the history.”
It’s interesting that originally (if my reading is right), we were going to have a small-plates style record for the whole of the Nephite history, and also a large-plates-abridgment for the the whole of the Nephite history. But now we have a small-plates start and a large-plate-abridgment end. Half of one, half of the other.