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.
To answer the original question:
Yes, gerunds all end with -ing, simply by definition. A gerund is, in Latin, a form of the verb which can be construed as (i.e. has functional characteristics of) a noun – it can act as subject or object of a verb, for example, or can take a plural ending. In English, the only category which meets this definition are "verbal nouns" or gerunds, which consist of a verb and a special -ing suffix which turns them into nouns. Although they look like present participles, they are morphologically separate, as we will see...
To answer the bounty question:
The gerundial -ing and the present participal -ing are, in fact, two different suffixes.
Let's start with the gerundial -ing. This is related to modern German -ung and modern Dutch -ing. It started life as a suffix forming nouns of action in Old English, usually written "-ung" - "gaderung" (gathering), "ceaping" (buying and selling). These gerunds were originally abstract, but even in Old English they started to develop into nouns of completed action, etc.: "bletsung" (blessing), "weddung" (betrothal). They subsequently developed plurals, and sometimes became concrete: "offrung" (offering). These uses all developed in the Middle English period, and by late Middle English they were well-established, particularly the gerundial use. Essentially, therefore, there never was a competing form for the gerundial -ing. It started off as "-ung" or "-ing" and continued in that form.
The present participial -ing, on the other hand, started off life as the -ende / -ande form that you refer to. It's related to modern German -end, and Swedish -ande. Even in the Old English period, -ende was often weakened to -inde, and by early Middle English there seems to have been a tendency to confuse "-inde" and "-inge" (this is particularly noticable in Anglo-Norman manuscripts of the 1300s.) Northern forms of the language retained -inde forms, however, though the distinction is not particularly obvious in Northern dialects, since both "g-dropping" and the tendency for "d" to be diminished after a preceding nasal consonant in an unstressed syllable (see, for example, the tendency to say "an" or "en" instead of "and" - "rock 'n' roll") means that either -ing or -ind may generally be produced as -in'.
It's possible that, in the later Middle English period, the development and prominence of the gerundial -ing, perceived as a quasi-verbal ending, also helped to strengthen the participial -ing. But their origins are quite separate, with participial -ing as a diminished form of -ande, while gerundial -ing has undergone few changes from its original form.
Information from my own knowledge, and from the OED's entries -ing, suffix1 and -ing, suffix2. Examples taken from OED, because they are excellent illustrations of the points.
Best Answer
As John Lawler has indicated in a comment, most negating words in Indo-European languages derive ultimately from the root *ne-.
In non-IE languages, negative words take other forms, eg Hebrew לֺא (lo' = "not") or אין ('eyn' = "there isn't") or Turkish yok ("there isn't") or Georgian არ (ar = "not").
And even in IE languages, there are exceptions which have arisen in other ways, eg French pas, ("not", "none") Welsh ddim (ditto), Danish ikke, ("not").