Once we understand precisely how music affects the mind, it will be clear what makes the 5th Symphony work, and it will be possible to create its functionality

Once we understand precisely how music affects the mind, it will be clear what makes the 5th Symphony work, and it will be possible to create its functionality as "the next obvious step". After all, it's not the sequence of notes that matters, it's the emotion that it creates. So yes, it's (4).