The word "four" was written as 亖 before Western Zhou and 四 appeared in late Spring and Autumn period. This alternative form was used to prevent confusion of 亖 and 二 or 三 in vertical writing. It was standardized in Qin dynasty.
The bronzeware style of the character featured a repositioning of those four lines inside 口; this later evolved into the combination used today of 口 (“mouth”) and 八 (“divide”) which meant a dispersal of breath. It could thus be said that four is a borrowed meaning for this character. The original sense is preserved in 呬 (OC *hrids), by adding an extra 口.
In modern Japanese, Japonic よ (yo) is more common than Sinitic し (shi, see Etymology 3) outside fixed compounds, which is similar to 七 (nana) but different from other numerals. The reason may be a superstitious connection to 死 (shi), or simply avoiding confusion with similar sounding 一 (ichi) and 七 (shichi).