For the larger passage to make sense, there must be multiple lands both “in the land south” and “in the land north”. Note the use of “all the lands” later on in the verse. In the following passage, we have another example in the text where a larger land is made up of distinct separate lands:
Therefore, the change in 3 Nephi 4:1 from the singular was to the plural were in the 1907 LDS vest-pocket edition is appropriate from a semantic (and grammatical) point of view. The 1911 LDS edition also made the change, perhaps independently of the 1907 edition. For some discussion of subject-verb agreement in the original text, see under 1 Nephi 4:4; for a general discussion, see under subject-verb agreement in volume 3. For each case of agreement (or nonagreement), the critical text will follow the earliest textual sources; thus was will be restored twice here in 3 Nephi 4:1.
Summary: Restore the two original uses of the singular was in 3 Nephi 4:1: “and began to take possession of the lands / both which was in the land south and which was in the land north”; the land south has multiple lands as does the land north, but the original text frequently allowed plural subjects to take the singular was as the verb form.