What is the purpose and significance of God’s declaration, as at Gen 1:4, that the things he has created are “good”?

This is a good illustration of the Bible’s understatement and tendency to require the reader to pay close attention in order to get the full message. The message here is not merely that God approves of his own creation, but that creation was, at first, good; but this changes, as we will see, with the Fall of Man in Gen 3. It is this change that makes the emphasis apropros.