In the printer’s manuscript, the word brethren was supralinearly inserted after beloved in darker ink, not by Oliver Cowdery (the original hand in this part of 𝓟) but instead by another hand. The word is definitely not in the hand of the unknown scribe 2 of 𝓟 or in the hand of Hyrum Smith, the two other scribes in 𝓟. In other words, this emendation does not appear to be the result of scribal proofing of 𝓟 against 𝓞. Rather, the intrusive brethren appears to be in the hand of John Gilbert, the 1830 compositor. (Compare this insertion with the words did excel that Gilbert inserted earlier in 2 Nephi 20:10, an Isaiah passage.) Here in Jacob 4:11, Gilbert may have felt that the word brethren was missing. Perhaps he had gotten used to reading “beloved brethren” in his copy and the use of beloved alone seemed like it lacked a following head noun.
In his various discourses, Jacob usually uses beloved brethren (19 times), but the bare beloved occurs two other times. Here in the book of Jacob, we have the following sequence of occurrences of “beloved (brethren)” in the original text:
According to the earliest textual sources, the bare beloved occurs three times in a row, first here in verse 11 and then the next two times (in verses 12 and 17). And the example in verse 11 is the first time the bare beloved occurs in the text, which may be the reason why John Gilbert decided to emend the text by adding brethren. But he did not continue making this emendation, thus leaving the two following cases of bare beloved unchanged. Notice also that there is no possessive pronoun before beloved in verse 11 or in verse 12; all the other examples in the book of Jacob are preceded by either my or our. Perhaps the lack of the expected possessive pronoun my may have also played a role in making “wherefore beloved” sound strange to Gilbert.
Thus the conscious addition of brethren here in Jacob 4:11 appears to be secondary. The critical text will restore the original bare beloved and maintain the two following cases of the same (in verses 12 and 17).
Summary: Remove the intrusive brethren in Jacob 4:11 that was apparently inserted by the 1830 compositor in order to make the text more consistent with previous usage.