It was discovered by David Phillips and his colleagues at the University of California, San Diego, who found that daily number of deaths of the 200,000 Chinese and Japanese Americans who died from heart attacks between 1973 and 1988 was 7% higher on the fourth of the month compared to the average for the other days in that week.
Four (四, formal writing: 肆, pinyin si4) is considered an unlucky number in Chinese and Japanese (as well as Korean) cultures because it sounds like "death" (死 pinyin si3). Some Chinese and Japanese hotels and hospitals do not use it as a room number
His hypothesis was that the peak was caused by stress induced by the superstition surrounding this number. Previous research had also shown a complementary effect, mortality falling before auspicious occasions and rising again afterwards.