It is the number of things that have a beginning, of things that are made, of material things, and matter itself. It is the number of material completeness. Hence it is the world number, and especially the "city" number.
The fourth day saw the material creation finished (for on the fifth and sixth days it was only the furnishing and peopling of the earth with living creatures). The sun, moon, and stars completed the work, and they were to give light upon the earth which had been created, and to rule over the day and over the night (Gen 1:14-19).