Love Live: Yon, nana, and not shi, shichi? (Counting from 1 to 9) In the anime Love Live, when the nine girls count from one to nine in Japanese, they say:


  Ichi ni san yon go roku nana hachi kyuu


And not:


  Ichi ni san shi go roku shichi hachi kyuu




What is the reason?
Four (四) being Yon not being Shi, this is because in Japanese Death is written as 死 and pronounced as Shi


  There are six unlucky numbers in Japanese. Traditionally, 4 is unlucky because it is sometimes pronounced shi, which is the word for death. Sometimes levels or rooms with 4 don't exist in hospitals or hotels. Particularly in the maternity section of a hospital, the room number 43 is avoided because it can literally mean "stillbirth". (死産 - shizan: 死 - death/to die and 産 - childbirth/produce).


Japanese superstitions > Linguistic superstition > Numbers > Unlucky Numbers



Seven (七) being Nana and not Shichi is sometimes in the same boat because of the same superstition above, however


  7, when pronounced with "shichi", sounds similar to the number four (四 shi). It is considered a good number since 7 symbolizes "Togetherness". 


Japanese superstitions > Linguistic superstition > Numbers > Lucky Numbers



Nine (九) in Japanese is Kyuu so i believe you are hearing it wrong. 

However it should be noted that 9 can be pronounced as Ku however in this instance it is like Shi as 


  Number 9 is sometimes pronounced ku — with the same pronunciation as agony or torture. Combs (kushi) are rarely given as presents as the name is pronounced the same as 9 4.


Japanese superstitions > Linguistic superstition > Numbers > Unlucky Numbers

and i have never been taught to use Ku1



1: only discovered Ku when i looked up Wikipedia for 7, in all my classes it has always been Kyuu