Here the original text shows a mixture of a finite that- clause and an infinitival clause. The subordinate conjunction that was dropped in the 1837 edition, probably as a result of Joseph Smith’s editing for that edition (although he did not mark the deletion in 𝓟). The current text thus has a full infinitival structure: “wherefore we know the manner to be true”. Another way of dealing with the mixture would have been to change the infinitive phrase to be to the indicative is: “wherefore we know that the manner is true”.
Elsewhere in the text, when the verb know is followed by a clause stating that something is true, we get the that- clause 11 times, as in this complicated example:
The infinitival clause, on the other hand, does occur, but only twice:
There is one other case in the text that refers to knowing something to be true, and in the original text that example has a mixture of the finite that- clause and the infinitival clause:
In this case, Joseph Smith changed the to be to is rather than deleting the that.
Either possibility for editing the text will work. Joseph Smith’s editing in one case changed to be to is (1 Nephi 1:3) and in another dropped the that (Moroni 4:1). The critical text, however, will in both cases restore the earliest reading, the ones with the mixture of clausal forms—thus “wherefore we know that the manner to be true” here in Moroni 4:1.
There is one other case of this kind of clausal mixture in the original text. In this third case, as in the one in 1 Nephi 1:3, there is some intervening material that seems to permit the switch from the finiteness of the that- clause to the use of the infinitival to:
For further discussion of this example, see under Mormon 6:6.
Summary: Restore the original that in Moroni 4:1, giving “wherefore we know that the manner to be true”; even though the result is a mixture of the finite that- clause and an infinitival phrase, to be, there is evidence elsewhere in the original text for such mixtures in clausal form.