what do you mean??

成语不一定仅限于四个字，或许字数会更多，甚至是两句话。

还有一个词是【谚语】（proverb）

（成语）http://zh.wikipedia.org/w/index.php?title=%E6%88%90%E8%AF%AD&variant=zh-hans

（谚语）http://zh.wikipedia.org/w/index.php?title=%E8%B0%9A%E8%AF%AD&variant=zh-hans

有的，但是非常少。绝大部分成语都是古代流传下来的，而且很多成语都有一个对应的典故。
也许你会问：为什么我们自己不去创造成语呢？
一个重要原因就是：在古代（我说的是从清朝到清朝以前），书面语言是用文言文，比较难懂，四个字的句子或者文章标题用得非常多。里面一些经典的四字词语经久日传就成为成语了（也有一些成语不是四个字的）。现代我们使用的是白话文，语言使用习惯已经是天壤之别，很难发掘出这么经典的词语了。即便某个人想出来一个非常经典的四字词语，得不到大家公认，仍然不能称为成语。
最早的成语应该是从春秋时期（2500－2700年前）开始出现的吧，这么长的时间里，留下来的成语就这么多。成语之所以叫成语，就因为经典！不是那么容易创造出来。

sorry，写了这么多，但是我没法把它们都翻译成英文，希望你能看得懂，多多了解中国文化。

Of course, all (Chinese) 成语 are made up by Chinese. ;)
However, newly developed ones can't (quite) be called that, since "成" means the phrase has to exist already. Given time, popular phrases like "正龙拍虎" may become a 成语, but most people would prefer ancient equivalents if there is any.

sometimes we do change 成语 and we make up our own words, but usually we won't call these new words 成语 ^_^
i think similar situations happen in other languages too

Good idea! A recommended book for Chinese 成语 is 成语典故，which collecting 成语 in these thousands of years. Those are historical stories and we modern Chinese people could never create a single 成语.

People often say something(four words) like a 成语, but actually it never can be a 成语. Because the real 成语 are created in or modified from the works of ancient great writers.

当然不,交流只是用很简单的词句.有些人喜欢说的成语也是很简单的,像,马马虎虎一心一意这些.

Sure, every language is changing as the time goes by. Different people don't always use the language as everyone else, So there are new idioms.

现在是白话文了。

For example，one year ayo， a chinese used a false photo to prove the existing of South China tiger. He wanted to get money by this way.Actually he just use software photoshop to create this tiger And finally he failed. And the people create a new idiom “正龙画虎” This idiom discribe someone who want lie to everyone but fail. Isn.t it funny

It is true that we change the existing 成语 into witty sayings, and u can see it in many ads. It can express ur meanings in short but really effective and favorable, but in recent years, many Chinese experts have protested against the witty sayings, afraid of misleading the young gernerations who r not educated enough to discern between right and wrong. And the new 成语, I think, is hard to coin, for 成语 have their all origins and is the classical Chinese expression to some extent, few people have the ability today, if one has, it's almost impossible to be confirmed and agreed by the whole nation.