For 4 and 7, Shi and Shichi are primarily used when counting.

So it would be Ichi, Ni, San, Shi, Go, Rouku, Shichi, Hachi, Kyuu, Juu. 

Kyuu and Jyuu can also be used for 9 and 10^^

Also, Ju would be spelled Juu since the u is doubled and Keu is spelled Kyuu or Ku.

Hope this helps. 

its SHI not si           and SHICHI not sichi     ^^

shi　＝　 し　 

There are variants for 4 and 7:

4: 'si' or 'yon'
7: 'sichi' or 'nana'

Depends on the context of where you're using them.