four
肆 centers on the number four, used primarily in formal or legal contexts as a more complex alternative to 四.
肆 is a formal character for 'four', historically derived from a pictograph of a display or market stall, later borrowed for the number four in official documents. The modern form combines 镸 (long) and 聿 (brush), but this is a graphic simplification; the original meaning is not directly related to these components.
Imagine a formal document where the number four is written with extra strokes to prevent forgery: the long 镸 and brush 聿 components make it look official and hard to alter.
For シ, think of a formal 'sheet' of paper with the number four written on it: sheet -> シ, and the character looks like it belongs on an official sheet.