This is a jukugo word, which usually means on'yomi readings from the kanji. The only exception to this is 四, which uses the kun'yomi reading, since the on'yomi reading sounds too much like the word for "death." Luckily you know these reading already, so there's nothing new to learn!
I have forty T-shirts.
That genius American is 40 years old.
Since reaching my forties, my entire body has been itchy.