成语不一定是四个字，但四个字的比较多。为何要凑成四个字？我认为主要是考虑了声音的美感。汉语非常讲究音韵美，比较注重语音的抑扬顿挫。这也得益于汉语的表达非常灵活，同样的意思既可以说得很复杂，也可以说得很简单。比如，“永远不会”可以说成“永不”。采用复杂的说法，还是简单的说法，就要考虑声音的美感了。

Cherry你好 成语大多是4个字的,至于为什么是4个字,我也不是很清楚,但是4个字已经能表达出一整句话的内容了. 没有什么特殊的significance. 我所见过的成语,即使不是4个字,也是由两个4个字的成语组成的,例如: 成也萧何,败也萧何. 意思是,一件事成功与否,都取决于一个人.

不知道你能不能看懂我用简单词汇写成的文章，其实上两位教师回答得很对。成语来自于中国的古代文学，中国古代的文学所用的语法和现带汉语语法有非常大的区别，最大的特点就是：中国古代文学中使用的语法可以用非常少的字来表达意思，那时的语法非常灵活，每个字的含义非常丰富。所以对于没有经过特别训练的人来说，要看懂古人写的书会有很大的困难。这就是为什么成语总是四个字的原因，当然也有一些成语超过了四个字。每个成语背后都有一个故事，但是随着时间的流逝，大部分成语已不再是它所具有的原始的含义，而是发生巨大的变化。中国学生在学成语时，他们会通过学习古文来学习它的含义，当然这些成语的含义仍是原始的含义。学生学习的大部分成语是通过记忆成语的含义和反复使用而被掌握。成语的结构和其中的每个字都是固定的、不能随意更改的，如果你那么做的话，别人就会听不懂你在说什么.

Hi,Cherry~Chengyu are mostly derived from ancient literature, but threr're also many Chengyu which are created from now on. Some chengyu have their own stories and meanings, and some others in fact come from daily life. All of Chengyu do not contain only one significance, it's different in different sentences. Like some English words have many significances. And now,we don't use too many Chengyu in oral chinese, mostly, we use chengyu in our writings. I think, Chengyu is difficult to learn. If you can use it correctly, your chinese must be perfectly well.

Hi Cherry, this is from Wikipedia. Chéngyǔ (traditional Chinese: 成語; simplified Chinese: 成语, literally "set phrases") are a type of traditional Chinese idiomatic expressions, most of which consist of four characters. Chengyu were widely used in Classical Chinese and are still common in Vernacular Chinese writing and Spoken Chinese today. According to the most stringent definition, there are about 5,000 chengyu in the Chinese language, though some dictionaries list over 20,000. They are often referred to as Chinese idioms or four-character idioms; however, they are not the only idioms in Chinese. Chengyu are mostly derived from ancient literature. The meaning of a chengyu usually surpasses the sum of the meanings carried by the four characters, as chengyu are often intimately linked with the myth, story or historical fact from which they were derived. As such, chengyu do not follow the usual grammatical structure and syntax of the modern Chinese spoken language, and are instead highly compact and synthetic. Chengyu in isolation are often unintelligible to modern Chinese, and when students in China learn chengyu in school as part of the Classical curriculum, they also need to study the context from which the chengyu was born. Often the four characters reflect the moral behind the story rather than the story itself. For example, the phrase "破釜沉舟" [Chinese-po4 fu3 chen2 zhou1.ogg Listen (help·info)] (pò fǔ chén zhōu) literally means "break the woks and sink the boats." It was based on a historical account where General Xiang Yu ordered his troops to destroy all cooking utensils and boats after crossing a river into the enemy's territory. He won the battle because of this "no-retreat" policy. Similar phrases are known in the West, such as "burning bridges" or "Crossing the Rubicon". This particular idiom cannot be used in a losing scenario because the story behind it does not describe a failure.