The latest excerpt from Yin Binyong’s book on Pinyin orthography covers how to write four-syllable idioms in Hanyu Pinyin (929 KB PDF). Here’s a key passage:
almost all four-character idioms can be broken in two halves, called y?jié ?? (language segments), on the basis of phonetic structure. The simple expedient of connecting the two y?jié with a hyphen then provides idioms with their own distinctive written form, and assures ease of writing and reading. It is also a simple rule for students of HP to master.
But not all four-syllable idioms follow this rule, as the reading shows.
This is a worthwhile reading for Mandarin learners, even if you’re not particularly interested in Pinyin. There are many examples of idioms here, all given in Hanzi, Pinyin, and English.