Korean idioms, generally derived from Chinese, consist of four Hanja characters. Many of these idioms has origins in mythical, historical, classical literature backgrounds. Though most of the idioms are 4 characters, there are some idioms composed of 2, 3, or 5 characters. Do note that most idioms are studied at a advanced level, though you might encounter them from intermediate level onwards. I will be adding to this list as time progresses.
박장대소(拍掌大笑): laugh aloud clapping one´s hands, cracking up