The noun mankind is consistently treated as a plural in the current text of the Book of Mormon, but in two passages the subject noun mankind took singular verb forms in the original text, twice here in Mosiah 4:7 and once in 1 Nephi 10:6: “wherefore all mankind was in a lost and in a fallen state”. In all three cases, Joseph Smith edited the singular be verb to the appropriate plural form, either were or are (see the discussion under 1 Nephi 10:6). All other instances of mankind take plural verb forms:
The critical text will restore the three instances where the word mankind originally took a singular verb form. For further discussion, see under subject-verb agreement in volume 3.
Summary: Restore in Mosiah 4:7 the two cases where mankind originally took singular forms of the verb be (“which ever was ... or which is or which ever shall be”).