What were the '400 years of silence'?

It is the period between Malachi and the beginning of the New Testament: approximately from Malachi (5th century B.C.) to John the Baptist.

It is called "silence" because no new Scriptures or canonical prophets are recorded in that interval.

Key events happen in that time: Persian → Greek rule → Hellenistic influences, persecution under Antiochus IV Epiphanes, the Maccabean revolt, Rome takes control (63 B.C.), groups like Pharisees and Sadducees rise/grow, expansion of synagogues.

And when the New Testament opens, John appears preparing the way (Malachi 4:5–6; Matthew 3:1–3).

Useful idea: God was "silent" in new revelation, but prepared the historical stage for the coming of the Messiah.