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The song (literally translated, "Night Letter") is out ahead of her comeback with her 4th full album this April. In the MV for the soft ballad, IU enjoys the sunlight, a stark contrast from her song title. 

IU will release a second pre-release on April 7 before making her full comeback with the release of her 4th album on April 21. Check out the "Through the Night" MV above!