When first learning Japanese numerals, you probably have learned that the numbers 4 and 7 have two different ways to express them in Japanese:
- 4 ＝＞ し / よん
- 7 ＝＞しち/なな
In some references and textbooks, they may leave it at that, which can leave you wondering which number to use when.
To begin with, the first word for each of the above (し and しち） is categorized as 音読み（おんよみ), or the “Chinese reading”. The second (よん and なな）is the 訓読み（くんよみ) or “Japanese reading”.
There are two rules when to use one reading and another, and I’ll illustrate them with examples.
Counting upwards
When counting numbers upwards, the 音読み is used.
- いち
- に
- さん
- し
- ご
- ろく
- しち
- はち
- きゅう
- じゅう
Counting downwards (or any other time)
When counting downwards, the 訓読み is used.
- じゅう
- きゅう
- はち
- なな
- ろく
- ご
- よん
- さん
- に
- いち
These pronunciations are also used for any other time (except counting up), such as when saying a number by itself or in some random order.
So why does Japanese have such a weird system where the way you say numbers 4 and 7 changes depending on the context?
I’m fairly certain the reason is something the everyday Japanese person doesn’t necessarily know, since they only need to learn the pattern and follow it. But for foreigners that are learning Japanese it’s a curiosity.
I came across this link which discuss this topic in good detail (in Japanese), and I’ll summarize the main point of it here.
The theory goes like this: the numbers 4 (し), 7 (しち), and 8 (はち) have the potential to be confused because of their similar sound, so in order to avoid problems the 訓読み for 4 and 7 (よん and なな） is used instead. The exception to this is when counting upwards, because with a easily recognizable pattern it’s difficult to mishear or misunderstand.
What about why ４時 is pronounced “よじ” instead of “しじ” ? From what I’ve read, the reason is the same – that しじ would sound too close to しちじ（７時）. I’m not sure why it isn’t “よんじ” but I think it’s because よ is also a valid 訓読み, as you can see on the table on this page.
Unfortunately, knowing the (supposed) reasons for these things doesn’t negate the fact we have to remember the irregularities. All I can say is “そのまま覚えればいいだろう”. In other words, just memorize it and get it over with (:
References
http://www2.chokai.ne.jp/~assoonas/UC180.HTML
http://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q1124005291