Popular romantic drama "Heartstrings" has just revealed 'Part 4' from its OST series, which includes the song, "I Guess You Don't Know".

The song played for the first time during the ending scene on the sixth episode, and was officially released on July 20th. "I Guess You Don't Know" became an instant hit, as it accurately captures the sorrowful feelings of Park Shin Hye's character, 'Kyu Won', who witnessed an intimate scene between her crush, 'Lee Shin' (played by Jung Yong Hwa), and another woman. The lyrics portray someone who loves another, but the person she loves will not realize or acknowledge her love for him. Hence, it fits perfectly with Kyu Won's secret love for Lee Shin. "I Guess You Don't Know" was sung by�M Signal, FNC MUSIC's newest project group. Check it out below! Source: TV Daily via Nate