How do you say the word four in Japanese?

四（よん、ｙｏｎ）. It is often written using the Arabic numeral 4, especially in conjunction with counters, as in　４人 （よにん、ｙｏｎｉｎ）, "four people".

You could either say "yon" or "shi," though yon is more commonly used because "shi" also means death in Japanese.