Numbers 4, 7 and 9

Japanese numbers 4, 7, and 9 have two readings because they come from two different origins: ancient Japanese and imported Chinese.

1. 音読み (Onyomi / Chinese Reading)

"The readings 'Shi' (4), 'Shichi' (7), and 'Ku' (9) are Onyomi, which originally came from Chinese." 

2. 訓読み (Kunyomi / Japanese Reading)

"The readings 'Yon' (4) and 'Nana' (7) are based on Kunyomi, which is the native Japanese way of counting." 

• 音読み： イチ、ニ、サン、シ、ゴ、ロク、シチ、ハチ、ク / キュウ、ジュウ
• 訓読み： ひ(と)、ふ(た)、み、よ、いつ、む、なな、や、ここ(の)、とお
• Onyomi : ichi, ni, san, shi, go, roku, shichi, hachi, ku / kyuu, juu
• Kunyomi : hi(to), fu(ta), mi, yo, itsu, mu, nana, ya, koko(no), too

数字	音読み (Onyomi) Chinese-derived	訓読み 　(Kunyomi)　 Native Japanese
4	Shi	Yon (from Yo-tsu)
7	Shichi	Nana (from Nana-tsu)
9	Ku / Kyuu (Both are Onyomi)	(Kokonotsu)

Number	Onyomi (Chinese)	Kunyomi (Native Japanese)
4	Shi (Used in months)	Yon (Commonly used)
7	Shichi (Used in time/months)	Nana (Commonly used)
9	Ku (Used in time/months)	Kyuu (Commonly used)

4 - 4月(shigatsu, April)　

7 - 7時 (shichi-ji, seven o'clock)、7月 (shichi-gatsu, July)

9 - 9時 (ku-ji, nine o'clock)、 9月 (ku-gatsu, September)

People prefer the Kunyomi 'Yon' and 'Nana' because the Onyomi 'Shi' and 'Shichi' are easily confused with other words or numbers.

In modern Japanese, we often mix these two systems to avoid unlucky sounds or to make the numbers easier to distinguish over the phone or in noisy places.

• 4 (Shi) = "Death" (Unlucky) → Yon is preferred.

• 7 (Shichi) = Sounds like 1 (Ichi) or 4 (Shi) → Nana is preferred for clarity.

• 9 (Ku) = "Suffering" (Unlucky) → Kyuu is preferred.