People are. Not "is", because the noun though singular is collective, with plural verb.
lack of knowledge. See note on Hosea 2:20.
knowledge. the knowledge [of Me].
thou hast forgotten. Reference to Pentateuch (Deuteronomy 32:18).
Related Commentaries of Hosea 4:6
Hosea 4:6
6 My people are destroyedc for lack of knowledge: because thou hast rejected knowledge, I will also reject thee, that thou shalt be no priest to me: seeing thou hast forgotten the law of thy God, I will also forget thy children.