The original manuscript here reads “yea and”. Oliver Cowdery accidentally dropped the yea when he copied from 𝓞 into 𝓟. The 1981 LDS edition correctly restored the yea, but the RLDS text here continues the reading of the printer’s manuscript.
Elsewhere the text has 300 occurrences of “yea and”, of which three are occurrences of “yea and he also” (1 Nephi 17:17, Alma 31:25, and Alma 31:38). The word yea is typically used in the Book of Mormon to comment on, amplify, or revise what has just been stated. Its use here in 1 Nephi 4:35 is wholly appropriate since it adds details to Zoram’s oath: he not only promises to go down with them, but he also promises to stay with them.
Summary: Based on the reading of the original manuscript, the yea before the second part of Zoram’s oath should be maintained.