What's distinguishes a chengyu (Chinese idiom) from a 4-character expression?

Question

I read the first two pages of 狐狸的窗户 (Húlí de chuānghù) = The Fox's Window (douban.com) on my Kindle, and encountered several 4-character phrases:

• 异口同声 (yì kǒu tóng shēng) = all with one voice (Baidu Baike; Chinese Tools)
• 精神恍惚 (jīng shén huǎng hū) = absent-minded (Baidu Baike; Chinese Tools)
• 胡思乱想 (hú sī luàn xiǎng) = go off into wild flights of fancy (Baidu Baike; Chinese Tools)
• 亮得刺眼 (liàng dé cìyǎn) = shine glaringly
• 一片一眼 (yīpiàn yīyǎn) = a glance at
• 望而生畏 (wàng ér shēng wèi) = be terrified by the sight of sth. or sb. (Baidu Baike; Chinese Tools)
• 心旷神怡 (xīn kuàng shén yí) = relaxed and happy (Baidu Baike; Chinese Tools)
• 擦去汗水 (cā qù hànshuǐ) = wipe the sweat

Some of these should be considered chengyu (成语, chéngyǔ), but others look like they just happen to have four characters. I'm wondering if there's a way of deciding which are chengyu and which are not.

Question: What's distinguishes a chengyu (Chinese idiom) from a 4-character expression?

It's possible this question is related to Are there any chengyu or xiehouyu that only mean their literal meaning? Maybe if it has a figurative meaning, then it's a chengyu, and otherwise not.

Answer 1

From

成语 versus 俗语 versus 谚语, what is the difference?

成語 (idiom) include "situation description" , "common sense", and "words of wisdom". Mostly came from known historical references, like poems, classical literature or famous quotes by historic figures .

4-character expressions are common phrases from unknown origin, just happen to have four characters, they are "idiomatic expressions" but they are not true idioms.