Therefore - More simply “and.”
They are foolish - Or, they act foolishly (see Numbers 12:11), not having that knowledge which would enable them to guide their ways with discretion.
Related Commentaries of Jeremiah 5:4
Jeremiah 5:4
4 Therefore I said, Surely these are poor; they are foolish: for they know not the way of the LORD, nor the judgment of their God.