To belatedly answer Charles Gilman's question about yo vs. yon: those lists of numbers in Japanese tend to gloss over a lot of things. Japanese actually has two full sets of numerals, one native and one originally borrowed from Chinese, but uses the same kanji for both (Japanese actually does this for a lot of things besides numbers: most kanji have both on-yomi, or Chinese readings, and kun-yomi, or Japanese readings, and may have more than one of each). The set used most commonly is the Sino-Japanese set: ichi, ni, san, shi, go, roku, shichi, hachi, kyuu, juu. In the modern language the native terms are more obscure (but do show up in certain restricted contexts, such as the 'tsu' counter and the first few counters for people) with the exception of 'yon' (4) and 'nana' (7), which are more or less interchangeable with the Sino-Japanese numerals when used stand-alone. In most kun-yomi compounds, though, the kanji meaning 4 appears as 'yo', sometimes doubling the following consonant (as in 'yottsu', 'four things'). I don't know the history of the language that well, but if I were to guess I'd say that 'yo' is the original form (or derived directly from the original form), and the '-n' was added just to the form that is used stand-alone and in compounds with on-yomi.

'Yon' is about as common as 'shi' (unlike most other numbers) because 'shi' is also an on-yomi for the kanji meaning 'die', and is therefore considered unlucky. This homophony was inherited when the kanji and their on-yomi were borrowed from Chinese, which has the same superstition about the number 4. Not sure why 'nana' is also an exception.

There is no kanji with the reading 'n'. All of them can function as a complete syllable. (Syllable-final 'n' in Japanese is sometimes referred to as 'syllabic N' but it really isn't, it just gets its own kana unlike the syllable-initial N, and makes the syllable long)