As you learn more and more Japanese, you will come across the apparently confusing Japanese counting system as the numbers get more complex. This is due to Chinese influence, as the counting system in Japan comes from Chinese tradition, grouping larger numbers in “myriads” (every 10,000, unlike the Western way of grouping in thousands (1000).
in Japanese, 乃rom 3 digits on, the numbers are counted as:

百 / “hyaku” /  “100”

千  / “sen” /  “1,000”

1万 /  “ichi-man”  /  “10,000”

Numbers from 20 to 90 are (“digit”)-jū
Hundreds from 200 to 900 are “(digit)-hyaku”
Thousands from 2000 to 9000 are “(digit)-sen”
Tens of thousands from 10,000 to 90,000 are “(digit)-man”

Keep in mind, that:
If written in Arabic numerals, they are separated by commas every three digits (the same way you have always done!)
And also, that:
in Japanese, when long numbers are written out in kanji, zeros are omitted for all powers of ten (i.e., “4002” is written as ” 四千二 “)
This can be can be useful especially if you are travelling around Japan and shopping around. It will really come in handy during your stay!