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?
When you have four hundreds, you have four hundred.
This word has a number in it, so it uses the kun'yomi reading for it. But the reading for 百 is the on'yomi reading. This is pretty normal when it comes to counting numbers in Japanese. You know both of these readings already. Hooray!