卌 is archaic and not used in modern Chinese; 'forty' is written 四十. It belongs to a small family of tally-numerals: 廿 (twenty), 卅 (thirty), and 卌 (forty), each adding one more vertical stroke.
memory
Count the uprights: 十 is one (ten), 廿 is two (twenty), 卅 is three (thirty), 卌 is four bars on one base - forty.
Components
ideograph
卌xì
forty
Four tally uprights tied by one horizontal bar - an abstract counting symbol for forty, not a picture of any object. Indexed under the ten radical 十 by the numeral tradition; not decomposable into independent parts.
Filed under radical 十 (shí, #24) by convention. 十 is not a separate component in 卌, so no strokes are highlighted.
