What's distinguishes a chengyu (Chinese idiom) from a 4-character expression? 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.
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 Mostly came from known historical references, like poems, classical literature or famous quotes by historic figures . 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.