Question: As I understand, Japanese numbers are divided into blocks of four, so while we would think of the number 89123889 as 89,123,889, in Japan they would think of it as 8912,3889 (八千九百十二万三千八百八十九). So English uses thousands and Japanese uses tens of thousands.

Is there any particular reason for this? Not that it matters either way, I'm just wondering at what point either culture chose to delimit numbers by thousands, or tens of thousands.

If you reply with Japanese, please use hiragana and furigana only, except for number kanji.

Answer: 
> 
> in Japan they would think of it as 8912,3889 (八千九百十二万三千八百八十九).
> 
> 
> 

Just FYI, no one in Japan writes numbers like that. Maybe you can find a museum or similar re-creation that does it when the tourists are watching, but retail, business, banking, government etc. use western formats.

The only place you will regularly see much smaller numbers written in kanji is on restaurant menus and addresses on new years cards, when the writer is going for an old-fashioned look, like Fraktur in Germany:

とんかつ：一千二百円

東京都品川区上町二丁目十七番9号

but the address is **much** easier to type as 
東京都品川区上町2-17-9

If you are learning this archaic writing method for the end purpose of reading pre-Meiji works in the original Klingon, keep studying. If you are learning modern Japanese for the purpose of working in or with Japan, you already know more than you will ever use. Move on to useful things.
To add to the previous answers, the rule is Arabic numbers for horizontal writing: 

> 
> 123456789
> 
> 
> 

and Chinese numbers for vertical writing. 

> 
> 一  
> 
>  二  
> 
>  三  
> 
>  四  
> 
>  五 
> 
> 
>