na-adjective, noun which may take the genitive case particle 'no', noun
Describes a person, attitude, or atmosphere that is rigidly formal, humorless, and overly proper. Often used critically.
彼の四角四面な態度には、みんなうんざりしている。
Everyone is fed up with his stiff, overly formal attitude.
四角四面の挨拶は抜きにして、気楽に話しましょう。
Let's skip the formal greetings and talk casually.