Most cultures that have developed number systems have also ascribed both power and meaning to numbers. As expressions of mathematical reality, they occupy an unusual sort of place in existence. Even absent complex numerology, certain individual numbers have gained special attention.
If one travels to Hong Kong, for example, they will find many buildings missing a fourth floor. Obviously the builders in Hong Kong aren't idiots. They build skyscrapers, so they've likely mastered basic arithmetic. The issue isn't counting; it's superstition. The number four is considered unlucky.
This fear comes not from some complicated numerology analysis or some ancient religious influence, but rather from something a bit more prosaic: the word four is rather close in sound to the word death in many East Asian languages.
Let's take a linguistic step back. The reason for this convergence is the influence of China on the various countries surrounding it. The might and majesty of the Middle Kingdom lead to the spread of its philosophies, language, and even religions all over East Asia. Even in the case of languages that are otherwise totally unrelated and unlike any form of Chinese (e.g., Japanese), Chinese words have been borrowed and/or modified to be brought into the language. Along with that borrowing came the similarity between death and four.
As I don't want to get side-tracked by linguistics, I'll just note when I say "Chinese" I'm actually referring to the collective Chinese dialects. Despite the fact they are often called dialects, most of these "dialects" are in fact separate spoken language as different from each other as most Romance languages are. The particular nature of the Chinese writing system, which is more meaning than phonetically based, however, allows them to all be written, for the most part, the same way.
Chinese itself is a tonal language. The tone pattern associated with a word changes meaning. The complexity of these tones will vary on the individual dialect. I mention this because four and death are not perfect homonyms in most Chinese dialects (nor were they in the ancient Chinese dialects that influenced all their neighbors). The tone distinguished them. They were still quite close. Some of the languages that integrated Chinese words were not tonal, and thus some of them now have true homonyms between death and four.
Now, there's a certain irony in all of this. There's something else that spread throughout East Asia and has come to heavily influence it--Buddhism. The fundamental tenets of Buddhism are referred to as the Four Noble Truths. It's slightly surprising to have a key aspect of such an influential religion associated with the unlucky number four. Buddhism, however, despite its current association with East Asia is actually South Asian in geographical, theological, and linguistic origin.
One might rightly wonder if this superstition is still common. Well, it's hard to say what percentage of people believe it, but I can testify from personal experience based on my time in Korea that it's far from an extinct there both in individual people or in general building numbering schemes.