“all fours ”是“四肢、四足”的意思。on all fours 趴在地上，匍匐着。

【例句】
You'll have to get down on all fours to clean behind the toilet.
你得趴到地上才能擦洗到马桶后面的区域。

He thought he heard a mouse under the bed, and got down on all fours to have a look.
他好像听见床底下有老鼠的声音，就趴在地上看了一下。