Four Hundred
numeral
When you have four hundreds, you have four hundred.
よんひゃく
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!
四百フィートは、およそ百二十メートルです。
400 feet is about 120 meters.
花見パーティに四百人もきた。
Four hundred people showed up at the cherry blossom viewing party.
わたしの人生は、四百ページの本にまとめられた。
My life was summarized in a four-hundred page book.