Yoji-jukugo are idiomatic expressions made up of four kanji characters. Japanese is normally written with three types of scripts: kanji, hiragana and katakana, but these idiomatic words are written in kanji only and with no kana between them.
Yoji-jukugo contain classical wisdom or morals in short phrases. In English there are words that people use which make them sound intelligent or denote higher education. Japanese also has words that educated people are more likely to know. Yoji-jukugo are such words. There are hundreds of yoji-jukugo. Some of them are not even familiar to the Japanese, therefore they are often asked at a school’s entrance examination to test one’s knowledge.