four
亖 represents the number four, written as a stack of two 二 (two) components, visually doubling two to make four.
亖 is an ancient form of the numeral four, constructed by stacking two 二 (two) characters. It was later largely replaced by 四 in common use.
Two stacked 二 (two) components: two plus two equals four. The character itself shows the arithmetic.
For シ, imagine the four horizontal strokes as four sheets of paper stacked together: sheets -> シ, and the stack makes four.