Although here in Jacob 4:7 the invariant form is the singular weakness, its meaning may very well be the plural ‘weaknesses’. As discussed under 2 Nephi 31:18, there is some evidence that the word witness, as a verb, seems to have twice stood for ‘witnesses’, the third person singular present, in the original Book of Mormon text (also see under 3 Nephi 16:6). Contextual evidence suggests that the noun wilderness may stand for ‘wildernesses’ in Alma 34:26 (see the discussion there). As explained under 2 Nephi 31:18, words ending in -ness commonly failed to take the expected inflectional -es ending in earlier English; such bare forms appear to have occasionally occurred in the original Book of Mormon text.
Don Brugger (personal communication) points out that a later passage in the book of Ether seems to uniformly use the form weakness to stand for ‘weaknesses’:
Note especially the use in this passage of weak things, a plural form, that seems to act as a plural substitute for weakness ‘weaknesses’. In other words, the clause containing weak things could be interpreted as meaning ‘then will I make weaknesses become strengths unto them’.
Summary: Maintain weakness in Jacob 4:7, with the understanding that this form may mean ‘weaknesses’.