Genesis 1:18

And to rule over the day and over the night, and to divide the light from the darkness: and God saw that it was good.

1. See comments for verse 16 regarding ruling over the day and the night.
2. God already divided the light from the darkness during the 1st day, so presumably he did more to accomplish this during this 4th day. Perhaps what he did during the 1st day was much more fundamental than I previously thought, since he hadn't yet created the sun, moon or stars; someone with more training in physics might have more technical thoughts on the subject. The work on the 4th day seems definitely related to light received by the earth.
3. God recognized and declared the work accomplished on the 4th day to be good.