The printer’s manuscript has the adjectival own before strength, while the 1830 edition does not. The 1908 RLDS edition as well as the 1981 LDS edition restored the own in this passage, under the incorrect assumption that the 1830 edition here derives from the printer’s manuscript, which is not the case from Helaman 13:17 through the end of Mormon. Nonetheless, the reading of the current text with the own is probably the reading of the original since the stronger tendency is for the early text to lose small words rather than to add them. But more specifically, as explained under Alma 5:14, the overall tendency in the text has been to accidentally omit own; but in at least four cases, own has been accidentally added because of a nearby own. And one of these errors was made by scribe 2 of 𝓟:
Scribe 2 of 𝓟 did not catch his error there, but Oliver Cowdery did when he proofed 𝓟 against 𝓞. One could argue that here in Mormon 4:8 we have one of those cases: scribe 2 of 𝓟 accidentally wrote “their own strength” because of the following “their own might”. Yet in this case, Oliver Cowdery did not add the own when he proofed 𝓟 against 𝓞, which makes one wonder whether the own was actually there in 𝓞. Moreover, we also have one clear case where the 1830 typesetter omitted the own:
Thus it is very difficult here in Mormon 4:8 to determine how 𝓞 itself read, with the own or without.
Elsewhere in the text, when the verb is boast, the modifier own occurs in five out of six passages (the one lacking the own is marked below with an asterisk):
Thus the odds favor the occurrence of own in expressions referring to boasting. Since the transitional probabilities are fairly balanced, it is probably best to accept the longer reading (given the stronger tendency to omit small words than to add them). Thus the critical text will accept the reading in 𝓟, “they did again boast of their own strength”, which means that in this instance the 1830 typesetter omitted the own. strength”; the 1830 reading without the own is apparently an error introduced by the 1830 typesetter.
Summary: Maintain in Mormon 4:8 the reading in 𝓟 with the own: “they did again boast of their own