Japanese numbers have some similarity with Tibetan numbers. Except for number 6, 7 and 8 [roku, shichi and hachi,] sound of first ten basic digits and multiple of tens are almost same. But when it comes to counting the material objects, Japanese system is very complex compared to Tibetans'.
Japanese counting system is one of the most intricate aspects of Japanese language. Based on the shape of the material objects, the counting system differs. Objects that can be perceived in length are counted as ippon, nihon, sanbon, yonhon etc., flat items in ichmai, nimai, sanmai, etc. For small things, it is ikko, niko, sanko etc. Then you have ikken, niken, sanken for buildings and itto, nito, santo etc. for big objects. There are series of counting system based on what item you are counting.
What interested me was the whole number 4 and 7 in Japanese. First ten whole numbers are counted as; ichi, ni, san, shi, go, roku, shichi, hachi, kyu and ju. But they have yon and nana, which is equally used for shi and shichi. I usually use shi and shichi when counting. Many Japanese also do the same. But there are many who use yon and nana.
There is a Karate dojo in my neighborhood where some twenty to thirty people practice together every evening, the participants are required to count from one to ten, sometime to twenty in turn in a loud voice for each karate punch, kick and other techniques being practiced. I observed that 70 percent uses shi and shichi, while the remaining people use yon and nana. Now, interesting things is those who use yon for shi also uses nana for shichi. Likewise the people who use shi use shichi only. It is never mixed or crisscrossed.
I surmised that early schooling and the family environment might be behind this habit. Other possible reason for those using yon and nana may be that they wanted to avoid the sound shi, because the sound shi is associated with death. Many Japanese tends to avoid number four in their daily life. But this I find it very odd, because the sound 'shi' is also associated with teacher, poetry, city, title etc. Why be so particular about death!
I raised this riddle with my Japanese friends and acquaintances to dispel my dilemma. Some said they have not seriously thought about this. Some said, shi and shichi are onyomi [Chinese way of reading] and yon and nana kunyomi [Japanese way of reading]. Some said yon is for [ 四] when Japanese character is written and shi is for Arabic number 4. But there was no consensus among the people I inquired as to the public preference of shi for yon and shichi for nana or the vice versa. I think there is more to it.