Which number is avoided in elevator buttons in some Asian countries?

The number 4 (四, pinyin: sì; Cantonese Yale: sei) is considered an unlucky number in Chinese because it is nearly homophonous to the word "death" (死 pinyin: sǐ; Cantonese Yale: séi). Thus, some buildings in East Asia omit floors and room numbers containing 4, similar to the Western practice of some buildings not having a 13th floor because 13 is considered unlucky. Where East Asian and Western cultures blend, such as in Hong Kong, it is possible in some buildings that the thirteenth floor along with all the floors with 4s to be omitted. Thus a building whose top floor is numbered 100 would in fact have just eighty one floors.

Houses and apartments that have 4 in the address number will sell for much less than ones without. Mentioning the number four to an ill family member is considered to be highly offensive, and giving anything with 4 on it to someone can be seen as a death threat.