In terms of chords and melody, Mega Man 4 - Boss Battle is significantly more complex than the typical song, having above average scores in Chord Complexity, Melodic Complexity, Chord-Melody Tension, Chord Progression Novelty and Chord-Bass Melody.
Mega Man 4 - Boss Battle is written in the key of F♯ Locrian. According to the Theorytab database, it is the 7th most popular key among Locrian keys and the 79th most popular among all keys. The F♯ Locrian scale is similar to the F♯ Minor scale except that both the 2nd and the 5th notes (G and C) are a half step lower. This means that the characteristic "home base" chord of F♯ Locrian has a dissonant diminished quality (f#˚), which makes it a very uncommon choice for songs. See the F♯ Locrian Cheat Sheet for popular chords, chord progressions, downloadable midi files and more!