Excerpt for Wikipedia article:

An elevator control panel in a residential apartment building in Shanghai with floor numbers 4, 13 and 14 missing. Floor 4 is missing because of the very similar pronunciation of “four” and “death” in Mandarin Chinese. Floor 13 is missing for many reasons. Floor 14 is missing because 4 is included in 14. Note that there is a “negative first” floor. ~wikipedia

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The Chinese word for four (?, pinyin: sì, jyutping: sei3), sounds quite similar to the word for death (?, pinyin: si, jyutping: sei2), in many varieties of Chinese. Similarly, the Sino-Japanese, Sino-Korean, and Sino-Vietnamese words for four, shi (?, Japanese), and sa (?, Korean), sound similar or identical to death in each language (see Korean numerals, Japanese numerals, Vietnamese numerals).

Special care may be taken to avoid occurrences or reminders of the number 4 during everyday life, especially during festive holidays, or when a family member is ill. Just mentioning the number 4 around a sick relative is strongly avoided. Giving four of something is strongly discouraged, and a popular idiom states that “you don’t do things in fours…”. Elevators in Asia and Asian neighborhoods will often skip the 4th floor or any floor whose number contains the digit “4” (as 14, 24, etc.). Military aircraft and ships will also avoid the number 4 (such as the South Korean and Taiwanese navies) due to its extreme negative connotations of death. April 4 is also considered an exceptionally unlucky day (much like Friday the 13th in the West). ~wikipedia