It has to do with spirituality, symbolism, and tradition. It symbolizes putting a part of four peoples spirit into woodcut. It is a tradition for all four people to put their spirit into it.
The Japanese woodcut prints were created by a collaboration between four people: an artist, a publisher, a printer and a block cutter. This print is known as ukiyo-e prints. It involved the division of labor as the Printer printed with water inks, the Publisher financed, the block cutter carved blocks and the artist drew the colors.
