Put a broiled fishhead in your entryway. Pile salt just outside your front door. Avoid cutting your fingernails after dark. Paint your house neon yellow. Do not whistle at night. What do all these odd actions/non-actions have in common? They are ways to ward off bad luck in Japan.
Japan is filled with such superstitions. While some are modern urban legends, many come from the animistic roots of Japan's religious beliefs. The many (millions of) gods and evil spirits in nature are capricious and mischievous. They must be appeased or driven away lest they bring death or misfortune to oneself.
Many superstitions surround the fear of death and suffering. The numbers four and nine are unlucky because they are pronounced in the same way as death (shi) and suffering (ku). Hospitals avoid the use of these numbers for rooms and floors. You'll also never find a set of four dishes in Japan. Plateware comes in sets of three or five.