(2016-06-14, 3:31 pm)FlameseeK Wrote: 4 kanji? How about 逆行性健忘症? Heard this one yesterday in a visual novel. But of course, 記憶喪失 is more commonly used. 
Well, that's a 6 kanji word and a 4 kanji word to be sure, but I think the question was really about 四字熟語. A lot of 四字熟語 don't break down into smaller words. (Although I wonder where the line is, some of the terms in that spreadsheet Yogert linked are not very 熟語 ...
Anyway...
(2016-06-14, 2:14 pm)yogert909 Wrote: I've been wondering about these. Wondering how helpful it would be to memorize a few at a sub n3 level. Katsuo has a spreadsheet broken down by frequency. But how common exactly are the most frequent ones?
A lot of the words are quite common, like 行方不明 ... although that's an example as I mentioned above of a word that's more... just a word. It's a pretty straightforward compound word that just happens to be 4 characters because it's a compound of 2 2-character words and isn't very idiomatic. You should know both 行方 and 不明 anyway, and that makes the compound obvious.
I have to question where that spreadsheet gets its frequency from too. 一生懸命 is incredibly common, as is 興味津々 (which are B+) while 誹謗中傷 is not that common. It's not that unusual or anything but I certainly wouldn't put it in a top-10 list, or I guess top 25ish list is what the A's amount to.
Maybe half of the A's and B's (not in order) are pretty common.
Standing out to me as incredibly common are the above mentioned 一生懸命、興味津々、行方不明 as well as 無理矢理、気分転換、露天風呂、一般常識、緊急事態、自分自身、正々堂々、中途半端、同一人物、東西南北、一人一人、無茶苦茶、滅茶苦茶 (those two are interchangeable, so neither is -that- common but add them together and they're pretty common), 危機一髪、日常茶飯 (usually seen as 日常茶飯事 which is not 4 characters but still), 意識不明、and 前代未聞.
I guess these all stand out to me because they're used in fiction, non-fiction, and conversation alike across a wide variety of topics. A lot of terms are used a lot more often... but only within a very specific context. A lot of stuff at the top of the spreadsheet is what you'd see in advertisements and labels, for example. Which is quite important if you're living in Japan, but not so much if you're trying to pass a test based on essays on general topics.
While we're on the 四字熟語 topic...
弱肉強食 is one of the first 四字熟語 that I learned, and I see or hear it a few times a year anyway (good ol' Jpod101...). It's usually used where in English we would say 'It's a dog eat dog world.' The more you are into 'hard boiled' or 'noir' kinds of fiction the more you'll see it. It's not very common in non-fiction or in daily life... unless you live a very rough kind of life I guess!
老若男女 is moderately common and worth studying because otherwise you'll never remember the unusual reading ... ろうにゃくなんにょ is the normal reading. Other readings aren't even in the 国語辞書, so whatever Rikai might say using other readings would seem to be an error.
安全第一 is extremely common, not in your reading, but mostly plastered as a header above safety rules in workplaces. I see it constantly in the background of dramas and movies, and occasionally in Anime or even on book covers. Oh, also in real workplaces on the news or documentaries. Actual use in a sentence is pretty rare, but if you watch any shows at all, you'll want to know it.