Yojijukugo (四字熟語) is a Japanese lexeme consisting of four kanji, or "Chinese characters". English translations of yojijukugo include "four-character compound", "four-character idiom", "four-character idiomatic phrase", and "four-character idiomatic compound". It is equivalent to the Chinese Chengyu.
Definition and classification 
Yojijukugo in the broad sense of the term simply means any Japanese compound words consisting of four kanji characters. In the narrow or strict sense of the term, however, the term refers only to four-kanji compounds that have a particular (idiomatic) meaning that cannot be inferred from the meanings of the words that make them up.
Non-idiomatic yojijukugo 
There exists a very large number — perhaps tens of thousands — of four-character compounds. A great majority of them are those whose meanings can be easily deduced from the literal definition of their parts. These compounds may be called non-idiomatic yojijukugo.
For example, the compound word 屋内禁煙 okunaikin'en "No smoking indoors." is a non-idiomatic yojijukugo. It is made up of four characters: 屋 oku building, 内 nai inside, 禁 kin prohibited, and 煙 en smoking. Alternatively, it can be regarded as consisting of two common two-character compounds: 屋内 okunai indoors, and 禁煙 kin'en prohibition of smoking. Either way, the meaning of the compound is clear; there are no idiomatic meanings beyond the literal meanings of its components. Below are a few more examples of non-idiomatic yojijukugo:
- 大学教育 daigakukyōiku (daigaku university + kyōiku education)
- 環境悪化 kankyōakka (kankyō environment + akka deterioration)
- 日米関係 nichibeikankei (nichi Japan + bei U.S. + kankei relations)
- 歴史小説 rekishishōsetsu (rekishi history + shōsetsu novel)
- 宣伝効果 sendenkōka (senden advertisement + kōka effect).
Note that 四字熟語 is itself a non-idiomatic four-character phrase.
Idiomatic yojijukugo 
By contrast, several thousands of these four-character compounds are true idioms in the sense that they have a particular meaning that is different from the meanings of the component words. An example of the highly idiomatic compound is:
- 海千山千 umisenyamasen (umi ocean + sen thousand + yama mountain + sen thousand)
Ocean-thousand-mountain-thousand means a sly old fox or someone who has had all sorts of experience in life so that s/he can handle, or wiggle out of, any difficult situations through cunning alone. This meaning derives from an old saying that a snake lives in the ocean for a thousand years and in the mountains for another thousand years before it turns into a dragon. Hence a sly, worldly-wise person is referred to as an ocean-1000-mountain-1000 person.
Many idiomatic yojijukugo were adopted from classical Chinese literature. Other four-character idioms are derived from Buddhist literature and scriptures, old Japanese customs and proverbs, and historical and contemporary Japanese life and social experience. The entries in the published dictionaries of yojijukugo are typically limited to these idiomatic compounds of various origins.
Chinese and Japanese origins of idiomatic yojijukugo 
The Japanese yojijukugo are closely related to the Chinese chengyu in that a great many of the former are adopted from the latter and have the same or similar meaning as in Chinese. Many other yojijukugo, however, are Japanese in origin. Some examples of these indigenous Japanese four-character idioms are:
- 合縁奇縁 aienkien (uncanny relationship formed by a quirk of fate)
- 一期一会 ichigoichie (once-in-a-lifetime experience)
- 海千山千 umisenyamasen (sly old dog of much worldly wisdom)
- 色恋沙汰 irokoizata (romantic entanglement; love affair)
- 傍目八目 okamehachimoku (a bystander's vantage point)
- 手前味噌 temaemiso (singing one's own praises; tooting one's own horn)
- 二股膏薬 futamatagōyaku (double-dealer; timeserver)
Examples of idiomatic yojijukugo 
- 一攫千金 ikkakusenkin (ichi one + kaku grasp + sen thousand + kin gold)
- making a fortune at a stroke. (Origin: Chinese classics)
- 美人薄命 bijinhakumei (bi beauty + jin person + haku thin + mei life)
- A beautiful woman is destined to die young.; Beauty and fortune seldom go together. (Origin: Chinese classics)
- 酔生夢死 suiseimushi (sui drunken + sei life + mu dreamy + shi death)
- idling one's life away; dreaming away one's life accomplishing nothing significant (Origin: Chinese classics)
- 羊頭狗肉 yōtōkuniku (yō sheep + tō head + ku dog + niku meat)
- crying wine and selling vinegar; extravagant advertisement (Origin: Chinese classics)
- 悪因悪果 akuin'akka (aku bad/evil + in cause + aku bad/evil + ka effect)
- An evil cause produces an evil effect; Sow evil and reap evil. (Origin: Buddhist scriptures)
- 会者定離 eshajōri (e meeting + sha person + jō always + ri be separated)
- Every meeting must involve a parting; Those who meet must part. (Origin: Buddhist scriptures)
- 一期一会 ichigoichie (ichi one + go life + ichi one + e encounter)
- (Every encounter is a) once-in-a-lifetime encounter (Origin: Japanese tea ceremony)
- 一石二鳥 issekinichō (ichi one + seki stone + ni two + chō bird)
- killing two birds with one stone (Origin: English proverb)
- 異体同心 itaidōshin (i different + tai body + dō same + shin mind)
- Harmony of mind between two persons; two persons acting in perfect accord
- 順風満帆 junpūmanpan (jun gentle/favorable + pū wind + man full + pan sails）
- smooth sailing with all sails set; everything going smoothly
- 十人十色 jūnintoiro (jū ten + nin person + to ten + iro color)
- to each his [her] own; So many people, so many minds.
- 自画自賛 jigajisan (ji own/self + ga painting + ji self/own + san praise/an inscription written on a painting)
- a painting with an inscription or poem written by the artist him/herself (as a non-idiomatic compound)
- singing one's own praises; blowing one's own horn; self-admiration (as an idiomatic compound)
- 我田引水 gaden'insui (ga own/self + den field + in draw + sui water)
- self-seeking; feathering one's own nest
- 唯我独尊 yuigadokuson (yui only + ga self + doku alone + son respect/honor)
- I alone am honored; holier-than-thou; Holy am I alone (Origin: Buddhist scriptures)
See also 
- Kanji Haitani's Yojijukugo 四字熟語 page, with about 3,300 examples translated into English.