The identical 'fourth year of Jehoiakim son of Josiah' date formula, tying Baruch's private oracle to the scroll-writing/burning narrative's own timeframe.
The message that Jeremiah the prophet spoke to Baruch the son of Neriah, when he wrote these words in a book at the mouth of Jeremiah, in the fourth year of Jehoiakim the son of Josiah, king of Judah, saying,
In the fourth year of Jehoiakim the son of Josiah, king of Judah, this word came to Jeremiah from Yahweh, saying,
This brief chapter closes the ch.40-45 narrative section, pausing the national story to address the one man who wrote it all down.