I think he meant he didn't change the paper for four days, not the water. I change on average once a week, to twice a week. Water is checked every day.
For me it turns into a "as needed" or at least once a week. Some snakes keep nice clean paper, and don't tip over the water, or poo on it for a long time. But once a week, everything gets changed anyway. If they tip the water over, or poo, then it gets changed.