Tetraphobia is a fear of the number '4' that is relatively common in East Asian countries. The reason for this is that the pronunciation of '4' in Mandarin is quite similar to the word for 'death'. As a result, many Hong Kong structures exclude all floors bearing the number '4' to avoid any connotation with death.
This is not, however, the only case of tetraphobia in East Asian countries. In Japan, the number '4' is also considered bad since it is pronounced similarly to the word'shi,' which signifies 'death'. As a result of this misconception, many Japanese hospitals lack a fourth level.
Don't be startled if you happen to be in Hong Kong and discover that a building appears to be missing a few stories! It's just superstition on the part of the natives.