In Chinese superstition, the number four is often considered unlucky because its Mandarin pronunciation, sì, resembles the word for death, sǐ.
This sound association is known as tetraphobia. It is strongest in some Chinese-speaking communities and can influence building design, numbering, gifts, and telephone numbers. Elevators may omit a fourth floor, or a floor labeled 3A may appear instead, although practices vary by country, developer, and resident.
The belief is linguistic rather than mathematical. The written characters and tones are different, but the similar sound makes the association memorable. Other East Asian languages can have related avoidance practices when their words for four resemble words for death.
Not every Chinese person regards four as unlucky, and the convention is not applied consistently. In some contexts, four can be treated neutrally or combined with favorable numbers. The superstition is therefore a cultural pattern, not a universal rule about the numeral itself.