Many idioms are often used in our life to express our meaning, because sometimes idioms can express what we want to describe better than vernacular Chinese, but you should know that idioms are not just four words, such as everything has its vanquisher. Many people or designers think this is a proverb, but it is actually an idiom. Isn't it strange? But it's true. Such multi-word idioms are often used in life, but we don't.
Now, I will tell you a common three-character idiom: How to stop a swift horse? It's a waste of wax and daydreaming. Centennial teachers put on airs, black sheep stand out from the crowd, birds make a fool of themselves, newborn calves, shuttlecocks are born, drummers brag and so on. What else is inseparable? For example, there are hundreds of steps that don't go into battle lightly, so there is nothing to boast about ... so there is no rule in the idiom that there are only four words.
No, when someone in life says this is a three-character idiom or a multi-character idiom, you can't joke. Of course, there are many four-character words, but they are just words, not idioms. Don't make a mistake. Keep learning, learning, and then better use, so as to carry forward the essence left by our ancestors and truly apply what we have learned.