I know the rough difference between a 成语 (4-character proverb)and a 俗语 (8-character or so saying), but what is a 谚语, and how does it differ from these?
I am also interested in 熟语。 How does this differ from these three?
成語 and 諺語 are well established common expressions , well known by the whole population. 俗語 is more often, limited to regional use.
俗語 : "有便宜唔好使頸" (Cantonese) : "If there's benefit to be have, don't be stubborn."
諺語: "神要毀滅某人，必先使其瘋狂" (western): " "Those whom the gods wish to destroy they first make mad."
熟语 (Well known phrases) include:
惯用语(common phrase), usually contain three characters, for example: "打秋風"，"拍馬屁" , 成語, 諺語, 俗語, 歇后语(two-part allegorical saying), for example: "和尚打傘，無法無天" and 格言(motto), usually from famous people, for example: "We have nothing to fear but fear itself." by Franklin D. Roosevelt.