The Shitennō are protectors of the four directions. They ward off evil, guard the nation, and protect the world from malicious spirits, hence the Japanese term Gose Shitennō 護世四天王, literally “four world-protecting deva kings.” Each represents a direction, season, color, virtue, and element. They originated in India but were later adopted into the Buddhist pantheon in China and Japan. They are venerated as temple guardians and protectors of the nation. In China, statues of the four are often placed near temple entrances, but in Japan, effigies of the four are more commonly placed around the central deity on the main altar. The four are commanded by Taishakuten, Lord of the Center. They are nearly always dressed in armor (yoroi 鎧), looking ferocious (funnusō 忿怒相), and carrying weapons or objects. They are also typically shown standing atop evil spirits (known as Jaki in Japan).
Shitennō iconography is related to the Four Celestial Emblems (dragon, red bird, tiger, turtle) of China, who also guard the four cardinal directions. In Japanese statuary, the Shitennō are almost always portrayed in animated warrior poses rather than static postures of ease or meditation...
Adapted from http://www.onmarkproductions.com/html/shitenno.shtml
See also http://www.onmarkproductions.com/html/buddhism.shtml