[katʁə vɛ̃ dis]
Let's break it down:
* quatre: [katʁə] - rhymes with "cot"
* vingt: [vɛ̃] - rhymes with "van"
* dix: [dis] - rhymes with "dee"
The "t" at the end of "quatre" is silent. The "x" at the end of "dix" is pronounced as [s], making it sound like "dees."
Therefore, the entire word sounds like "cat-ruh-van-dees."