Why? The pronunciation of "four" is close to that of "death" in Chinese.
If you use the number four on happy occasions such as wedding, especially in the southern part of China or in the countryside, you are likely to be criticized behind your back for failing to understand basic manners. Hospital and hotels normally have no fourth floor, and the numbers in the elevator just skip from three right to five. And in some cases the price of an apartment on the fourth floor is usually lower.
Source: http://www.chinaonyourmind.com/chinese%20culture/chinese%20customs.htm
