Why does Genesis say that God made light and darkness on the first day but did not make the sun moon and stars until the fourth?

There are plenty of alternative explanations. One is that parts of The Bible are not to be taken literally but are to be taken as still conveying truth. One theory is that Genesis 1 is written in a form of Hebrew poetry based on parallels. The first three days involve God separating things. The next three days involve God filling things. The finaly day is to start the tradition of the Sabbath. The important thing is that God created all of these things, not the exact order He created them in.