Verse 4 is pivotal in understanding both the way Nephi uses Isaiah, and why Nephi has not taught the Jews the "works of darkness," which has apparently included the "manner of prophesying of the Jews" that would be required to understand Isaiah (verses 1-2.)
Nephi addresses his future audience, the descendants of the house of Israel. Having cited Isaiah prophesying to and about Israel, Nephi now ties the historic Old World Israel to the New World Israel. In so doing, Nephi is clearly noting that the message of Isaiah will pertain to the New World Israel as well as the Old World Israel. Nevertheless, he also notes that while Isaiah's message is clear to one with the spirit of prophecy, it was nevertheless not sufficiently clear either to the Old World nor the New World Israel. In order that the New World might plainly understand the message Isaiah had for them, Nephi will expand on some themes in Isaiah to underline and support his own vision.
Nephi will give his own prophecy. He announces that this is not an exegesis of Isaiah, but rather Nephi's prophetic message, that happens to expand on Isaiah's themes. Nephi, however, prophecies in plainness. This must be contrasted not only to Isaiah but also to Lehi. Those two were of the Old World prophetic tradition. Nephi is consciously altering that tradition and creating a new line of prophetic utterance that is couched in "plainness" rather than what Nephi has considered the more obscure, and therefore little understood, style of Old World prophecy.
Narrative: The movement from subject to development has taken a side trip here. Nephi begins with the purpose of elaborating upon Isaiah, but enters here on an aside directed to his future audience. Once again while the information is appropriate and vital, the point is in the lack of structure of the narrative. This aside is triggered by the discussion of the differences between Old World and New World prophetic forms in Nephi's people. It is not a structured part of what might be considered a argument based on Isaiah, it is rather an explanation as to why such an argument need be given.
Where the younger Nephi (in the recast speeches) was precise and deliberate. this aside comes as an indication that the information is coming from the stream of consciousness triggered by context, not by developed argument.