https://en.wikipedia.org/wiki/Numbers_in_Chinese_culture From this article: The word for "eight" (八 Pinyin: bā) sounds similar to the word which means "prosper" or "wealth" (發) Number 4 (四; accounting 肆; pinyin sì) is considered an unlucky number in Chinese because it is nearly homophonous to the word "death" (死 pinyin sǐ).

因为“八八大顺” 这个成语里有八，所以八就成了吉利的象征 而四，，，我的理解是和死（death）同音吧