Why do common swear words have four letters?

As a non-native speaker I always wondered why most (common) swear words have four letters. I know this is shifting and more words are araising and traditional swear words lose their "harshness", but where does this "4-letter" thing come from? Is it historical or did it just evolve?

I am not sure if there is a definitive explanation.
The Wikipedia article simply mentions:

The "four-letter" claim refers to the fact that a large number of English "swear words" are incidentally four-character monosyllables.
This euphemism came into use during the first half of the twentieth century.1

This thread refers to slang as another form of "emotional speech":

It is a known fact that when an english speaker wants to make the most emotional statements, he resorts to Anglo-Saxon words of one syllable

That being said, "four letter words" is also an expression of its own:

Occasionally the phrase "four-letter word" is humorously used to describe any word composed of four letters.
This is the case when used to mean the word work, alleging that the speaker's or writer's audience treats work as unpleasant.

Related to what VonC said about most of them being monosyllables: the number four may just be due to the syllable structure of English. The typical pattern (more so in words derived from the older stratum of the language, Anglo-Saxon, which is the source of most of the common profanities) is CVCC or CCVC (where C=consonant and V=vowel), yielding four letters.

If our language had a structure more like Japanese or Hawai'ian (both of which are basically CV with some exceptions), then we'd probably speak of "two-letter words."