As explained under Alma 24:4, the original text has only the plural preparations in the phrase “to make preparation(s)”. Yet there has been a tendency to replace the plural with the singular, as here in 𝓟 for Mormon 4:6. In this instance, 𝓞 probably read in the plural, which is how the 1830 compositor set the word. But scribe 2 of 𝓟 miswrote the word as peperation, with the result that when Oliver Cowdery proofed 𝓟 against 𝓞, he supplied the missing r, giving preperation. But the defective spelling seems to have prevented Oliver from noticing that scribe 2 of 𝓟 had written the word in the singular, so in his correction Oliver did not add the plural s. The 1874 RLDS edition also made the change to the singular, and that reading has been retained in the RLDS text (supported by the singular preperation in 𝓟). The critical text will maintain the plural preparations, the consistent reading of the Book of Mormon text for this phrase. For another example of this change to the singular, see nearby under Mormon 2:4.
Summary: Maintain in Mormon 4:6 the plural preparations, the 1830 reading; the Book of Mormon consistently prefers the plural phraseology of “to make preparations”.