This is background information about the prophets of God, and the 400 years that God did not send them. (See: [[rc://en/ta/man/translate/writing-background]])
This could be translated as: “his people, the Israelites” or “his people, the Jews.” But only include the added information if it is not clearly understood who these people were.
Other ways to say this are: “Then 400 years of silence passed” or “Then there were 400 years of silence.” The last Old Testament prophet, Malachi, has prophesied 400 years before.