In this passage Joseph Smith twice deleted the conjunction that in the context of an immediately following if. In the first case, the text originally read “behold that if ”, which is quite awkward for modern English readers. Even so, there is evidence that the original text of the Book of Mormon allowed that to follow behold, as in Moroni 8:22: “for behold that all little children are alive in Christ”. For another example, see the discussion under Jacob 5:24.
There is less-prescriptive motivation for deleting the that that follows “I say unto you” in Mosiah 4:6. The original text had 14 occurrences of “I say unto you that if ”, including four more from king Benjamin:
In all 13 of the other occurrences of the phrase “I say unto you that if ”, Joseph Smith left the that in the text. The critical text will, of course, restore the that here in Mosiah 4:6 (as well as the more difficult one following behold in the previous verse). For a complete discussion of the omission of the subordinate conjunction that for various verbs, see under THAT in volume 3.
Summary: Restore the two instances of the subordinate conjunction that in Mosiah 4:5–6: “for behold that if the knowledge ...I say unto you that if ye have come to a knowledge”.