You may have noticed that certain floors in an apartment building are 'missing'; often there are no fourth, 14th, 24th, 34th floor, etc. This is because 'four' sounds like the word 'death' in Cantonese, making it unpopular with both tenants and landlords, who will also often number an apartment 3A or five to avoid using the unlucky number. Some buildings have no 13th floor (as per Western tradition), making the floor after the 12th the 15th.
The concern over the number four is cultural rather than related to the metaphysical and some traditional fung shui masters deride the superstition.
In the Flying Stars school of fung shui, the number four is associated with romance and academic studies. The negative numbers in this system are two and five, the former associated with illness and the latter with catastrophes. Indeed, there are buildings that as well as not having the fourth floor, also omit the second and fifth, which means the second floor is known as the third and the sixth is only three levels above the ground floor.
The cultural beliefs associated with the number four are so deeply entwined that even phone numbers or car registration plates that feature it are generally avoided. Perhaps the best advice for those wary of the number four is to write off the fear as irrational.