Exodus 20:5 (King James Bible) for I the LORD thy God am a jealous God visiting the iniquity of the fathers upon the children unto the third and fourth generation of them that hate me
The printer’s manuscript has the singular generation, in agreement with the corresponding reading in Exodus 20 of the King James Bible. The 1830 typesetter (accidentally, it would appear) changed generation to the plural generations, which all subsequent editions have followed. One could literally translate the Masoretic Hebrew as “unto the thirds or unto the fourths”; the plurality in the Hebrew refers to the plurality of the individuals themselves, not to the collective generations per se.
In similar biblical passages, the King James Bible consistently uses the singular generation (and always in italics, as in Exodus 20:5):
The plural reading in the current text for Mosiah 13:13 is clearly an error in transmission; the critical text will restore the singular.
Summary: Restore in Mosiah 13:13 the singular generation in the phrase “unto the third and fourth generation”; the reading of the printer’s manuscript agrees with every King James citation of this same phraseology.