This word has number four in it, and uses the kun'yomi reading for that kanji (よん). The reading for 百 though, is the on'yomi reading (ひゃく). This is pretty normal when it comes to words containing four in Japanese. You know both of these readings already. Hooray!
Please make four hundred copies of this document.
There was an incident where 400 people went missing overnight in the city of Seattle in the US.
How did you spend four hundred dollars on a toothpick?