First of all, the dragon, the white tiger, Suzaku Xuanwu, not the black dragon. The four gods, also known as the four elephants, can be the four seasons or the stars in the sky. Mr. Feng Shui applied the four elephants to the terrain, using their images and movements to explain the terrain, and attached good and bad luck.
See Guo Pu's Funeral Sutra for relevant information.