“Then God said, ‘Let there be lights in the expanse of the heavens to separate the day from the night, and let them be for signs and for seasons and for days and years;’” (Genesis 1:14, NASB)
Day four focuses on the heavens, the sky. God creates again by His sovereign command. He separates the day from the night, as opposed to the light and darkness earlier. This is done with lights in the expanse. We see again the phenomenological language used to describe the position of the lights – in the expanse, or sky. The same is used by us today when we look at “the stars in the sky” (for more see Genesis 1:6, under “Lastly”).
God reveals that He is the one who created the lights in the sky. Before His revelation to Moses ends, God reminds us of this and points out that they should not be worshipped, only the Creator should be worshipped (Deuteronomy 4:19). Even with all these beautiful and magnificent creations, though they are wonder-full, God is concerned with us above all (Psalm 8:3-4; Matthew 10:31; Matthew 12:12).
These new lights are for signs to us. Here God is again organizing the earth to sustain life. They will enable seasons, days and years that we may organize our lives. In Genesis 8:22 our faithful God promises that these signs will continue until the earth is no more (Revelation 21:1).