As discussed under 2 Nephi 11:8, the critical text will restore here in Mosiah 4:28 the relative pronoun that was deleted in the 1920 LDS edition. As pointed out in that discussion, there are three passages that support the use of a relative pronoun (such as that, which, or who) in wh-clauses headed by whoso(ever). The edited phraseology here in Mosiah 4:28 is somewhat awkward to read, although it does agree with the usage in 3 Nephi 18:13 (“but whoso among you shall do more or less than these are not built upon my rock”). We can also find such usage in the King James Bible:
A less awkward reading for 3 Nephi 18:13 would be “whoso among you that shall do more or less than these” and for Acts 13:26 “whosoever among you that feareth God”. The Book of Mormon text generally favors the inclusion of the relative pronoun in these wh-clauses, in accord with modern English usage.
Summary: Restore the original reading of the text in Mosiah 4:28 (“whosoever among you that borroweth of his neighbor”); the editing in the 1920 LDS edition introduced a rather awkward and now archaic type of wh-clause (“whosoever among you borroweth of his neighbor”).