I believe that Bush cares too much about the troops safety to expose them to the risk without taking advantage of the darkness. We are making more attacks against the Iraqi radar sites these days and that is surely part of the preparations for the main attack.
If we can eliminate most of his radar sites before the attack, we may put it off those four days but I wouldn't count on it. We would have to be certain that we didn't need the darkness before we would discard it. Most of our offensive doctrine is predicated upon the usage of the new moon to protect our planes from visual sighting and that is a very beneficial policy not to be discarded lightly.