This in from Harvey at Japan Newbie:

Japanese are always impressed when a foreigner (or even another Japanese for that matter…) can work a four-character phrase into daily conversation.

Here are a few that I have run across while reading stuff that are good to know. Complete definitions and usages of these can be found in your trusty electronic dictionary, and shorter definitions are also available on the popular ALC and BREEN dictionaries.

ikki ichiyuu

Literally, the characters mean one happiness, one “despair”. The phrase basically means that one is alternating experiences happiness and sadness. I picked this one up when a friend emailed me about how things were going with a new baby. Sometimes it’s so fun and exciting, but other times can be really hard, like when the baby is crying for seemingly no reason through all hours of the night. (ugh!)

tanjyun meikai

Literally, the characters mean “simple and clear”. This one I came across in the dialog of a video game… (who says games aren’t educational!) It was used by a villain to insult a hero character who was hard headed and very one-dimensional. Basically the villain could read the hero “like a book”. This phrase basically means “plain and simple”, and can be used in a positive or negative sense. For example, tanjyun meikai dieto would be, a diet that is clear and simple to understand. No complex counting of calorie units there!

boujyaku bujin

This one is used to describe someone who is insolent. They may be a new employee in a company, but always gives their opinion in meetings, even disagreeing with their superiors. This phrase applies even if they are correct. So it’s sort of a combination of someone who is bold, yet naive and doesn’t realize it. It’s a bit difficult to explain, but if you imagine the situation of the fresh employee speaking up and going against the flow in a meeting situation, I think you’ve got it.

That’s enough for now! Does anyone have any four-character phrases they want to share? - Harvey

Here is the link to Japan Newbie.