Tetraphobia

The word is Greek on the surface and Sinitic underneath. Tetra is four and -phobia is fear, the same pattern that gives triskaidekaphobia, but nothing about the Greek explains why this particular digit. The reason sits in the sound. Mandarin four is 四 sì and death is 死 sǐ, one syllable on two tones. Cantonese pulls them closer, where four is sei3. Japanese reads 四 as shi, which is also the reading of 死, and speakers switch to the native yon when a count could be misheard. Korean sa and Vietnamese tư carry versions of the same overlap.

That is the whole mechanism, and it is why the avoidance is regional. The taboo travelled with Chinese vocabulary into Japanese and Korean and stops at the edge of the languages that borrowed it. A speaker of a language where four sounds like nothing in particular inherits nothing in particular.

The word covers a social convention rather than a diagnosis, in exactly the way triskaidekaphobia does. Lift panels, hospital wards, flight numbers, phone numbers and product names all show the effect, and the documented cases with dates and places are collected on the unlucky numbers page rather than here.