There are various ways of counting in
Japanese. In karate we use numbers for moves, name of kata, grades, etc.
but some people may be confused its usage.
Generally 4, 7, and 9 have two ways of
reading, and it is said that you can use alternative readings when you want to
avoid unlucky feeling or confusion.
4: Shi --> same pronounce as ‘death’; unlucky feeling -->Use Yon
7: Shichi --> similar pronounce to ‘Ichi’; confusion --> Use Nana
9: Ku --> same pronounce as ‘pain’; unlucky feeling --> Use Kyu
(1: ‘Sho’ is used specially for kyu/dan
etc.)
This
is just one of many interpretations. For the usage in your dojo, follow sensei’s instruction.