I passed TOPIK I and want to prepare for TOPIK II to reach Level 4 or higher?

Reaching Level 4+ on TOPIK II mainly comes down to vocabulary, grammar range, and the writing (쓰기) section, which trips up a lot of intermediate learners. Build a steady core of intermediate vocabulary and grammar from a series like Sejong Korean or the TOPIK-specific books, and drill with past papers — all the real exams are free on the official site (topik.go.kr), so timed practice with them is the single most effective thing you can do. For writing, practice the 200–300 character essay using set structures and get corrections from a tutor or language exchange, and for reading, work on speed and skimming since time pressure is the main obstacle. Free YouTube channels help, but consistent past-paper practice plus writing feedback is what moves people from Level 3 to 4–5.