Hi,
I'm of the opinion that idiomatic yojijukugo like 弱肉強食 (jakunikukyoushoku) are proverbs and should be tagged and glossed as such, while Rene has put forward the opinion that since they are nouns, they can't be proverbs.
What is a yojijukugo? Daijs says " "字4字で構成される熟語。特に、「以心伝心」「不言実行」など、成語として用いられるものをいう。" I.e. all 4 letter kanji compounds count, but it's esp. used to refer to the one's that are seigo, which daijs gives as "古くからひとまとまりで慣用的に用いられる言葉。ことわざ・格言の類。成句。"
What is the English definition of a proverb? Random House: "a short popular saying, usually of unknown and ancient origin, that expresses effectively some commonplace truth or useful thought"
Merriam-Webster: "a brief popular saying (such as “Too many cooks spoil the broth”) that gives advice about how people should live or that expresses a belief that is generally thought to be true"
From this it seems to follow that the reason a noun can't be a proverb in English has little to do with the definition of a proverb, and more to do that you can't string together nouns in the same way you can in Japanese. If we translate the yojijukugo literally we end up with noun phrases like "weak-meat-strong-eating" and "one-stone-two-birds" which still probably would be classified as proverbs, albeit in mangled Tarzanese-English.
The other wa-ei dictionaries also often gloss yojijukugo as phrases rather than nouns. 一期一会 (ichigoichie) for example is "Treasure every meeting, for it will never recur" in Kenkyusha's Shinwaeichuujiten (and something similar in eijirou). 色即是空 (shikisokuzekuu) is defined as "All is vanity./ All visible things are vain." in prog. Actually some EDICT yojijukugo are also phrases: the full gloss for 敬天愛人 (keitenaijin) is ' "Revere heaven, love people." (favorite motto of Saigo Takamori, 1827-1877)'.
In the end, I think it's ultimately much more helpful for our end users if we gloss the idiomatic yojijukugo as "[exp, n][proverb]" rather than just "[n]" and provide [lit]'s where it makes sense.
Cheers,
Marcus