Korean 4 is the fourth volume of the Korean Language textbook series by the Language Education Institute of Seoul National University, written for adult learners who have a knowledge of Korean with about 600 hours of classroom time or the equivalent. The text aims to develop necessary communicative skills in various everyday social situations.
As far as classroom language textbooks go, it isn't too bad. My only complaint is that there are no grammar explanations (in Korean, English or any other language). There are only example sentences. I understand this is intended for classroom use and the teacher should explain all the grammar, however, sometimes I missed class, couldn't follow the teacher, or tried to review on my own at home and some explanations would have been immensely helpful.