In his editing for the 1837 edition, Joseph Smith did not normally replace the restrictive relative pronoun that with who, but for this example in the 1840 edition it appears that he did. (It is, of course, possible that the replacement was made by the 1840 typesetter.) There are two more instances of the replacement of that with who later on in this chapter (which suggests that these changes in the 1840 edition were intentional):
All three of these changes in Mosiah 4 were also made in the stereotyped plates for the second printing of the 1852 LDS edition (undoubtedly by reference to the 1840 edition). The critical text will restore the original that in all three of these instances. For a complete list of the cases where the relative pronoun that has been changed to who(m) or which, see under which in volume 3.
Summary: Restore in Mosiah 4 the relative pronoun that in the three cases where who replaced that in the 1840 edition (in Mosiah 4:7 and twice in Mosiah 4:24).