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.