You’d think that, following the logic of the first three numerals (一, 二, 三), the symbol for “four” should be written as 亖. And you’d be absolutely right. However, nowadays this form of “four” is considered archaic. The historical forms of the character give us no clue why the (easy) four strokes got replaced by the somewhat more complicated symbol, which looks like a square window with two curtains. Unless it was invented by an elementary math teacher who wanted his pupils to remember that two (curtains) squared is four.
Naturally, the Chinese hand sign for “four” is four extended fingers.
There is another hanzi with the same meaning, 肆, which is mainly used for financial purposes. You’d never guess.