Textual: These are the words of Benjamin. It is an embedded text from the source. While we cannot be certain that there is no abridgement of the discourse itself (neither here nor in future examples) we may presume that the embedded discourse tend to remain intact, and that any alterations by Mormon will be noticeable as his interjections. The embedded speeches are fairly clearly marked with noticeable changes in subject and or tense to represent the speaker in the original document.
Anthropological: Verse 4 is directly tied to the conception of language and learning the words of the fathers. In particular, Benjamin highlights the importance of the brass plates, but ties the importance of the plates to the importance of learning "language." Lehi was "taught in the language of the Egyptians therefore he could read these engravings [brass plates]."
In the context of Benjamin's discourse to his sons we learn that the records are important, and that what they contain should be learned. To pass this information on, Lehi had to learn the "language of the Egyptians." In the context of what must have preceded this specific discourse based on Mormon's abbreviation, this discourse is set in the context of learning language for the purpose of reading the brass plates (as well as the large plates). Within that context, Mormon's inclusion of the learning of language must be seen as important information which provided the necessary background for this discourse. That necessary background was not a child simply learning the language that was spoken around him, but rather the very specific teaching of a language with which they would not otherwise be familiar.
Concerning the language of the plates, we are still in the conundrum of the separation of language from script, where there exists some possibility that the script represents a different language (commonly supposed to be Hebrew language and Egyptian script). Of course the script could also represent "Egyptian" language. In any case, the substitution of one script for another over a known language is not a particularly difficult task. Much more difficult is the change in language itself, and I prefer this second reading of the situation. Regardless of the current spoken language, the language (plus script) was not natural to the learning of the sons of Benjamin in Zarahemla, and therefore required specific learning so they might be both "men of understanding" and be able to read the brass plates just as Lehi did (and by extension the large plates, which may be presumed to have been created on the model of the brass plates with which Nephi was clearly familiar and emotionally and spiritually attuned).
It has been noted previously, but bears repeating, that the experience with the brass plates was a watershed event in young Nephi's life, and the plates would have had even greater import for him than for his father. The fact that the brass plates were engraved on metal and that they required "the language of the Egyptians" to read both provide the likely model for the plates that Nephi forged, both for creating a record on metal as well as the language (which would have been sacralized for him, whatever the possible pragmatics of its origin on the brass plates). It is even quite likely that the physical dimensions of the plates of Nephi would have followed the model of the brass plates.