The fourth wall is where a piece of media engages directly with the audience. This could be an actor looking directly into the camera and saying, to the audience "I know I'm just a character in a movie," or a character in a video game trying to convince another character that they're "a character in a simulation, a video game". The media, and the characters within it, become self aware. Often creepy, sometimes observational, or used to help drive the story home with more emotional impact.

Song is something that does this - probably far more commonly than other forms of media. There's more songs than movies, more songs than games, so it comes to reason that the averages result in this. The fourth wall being broken in song invites people into the artist's life, and we get something much deeper.
That is also why I love reading hand written lyrics. I know they are usually "secrets" and commercial in confidence before being turned into a song (even if they're commercially valuable only to the self) - but I guess that makes them a poem before they are song.
I remember "reading" a book of Curt Cobain's journals that I borrowed from the public library. It was such an an unusual format for a mass market book. I was probably in my early-teens, and thoroughly interested in writing. I was glad I had the opportunity to experience it, as hand written things tend to have much more intimate appeal than something typed and polished. It is an irony that I am not hand writing this post, given that I founded the hand written community on Hive.
More recently, I have been diving deeply into Florence Welch, and her book, "Useless Magic" filled with a whole bunch of collage, mixed media, scribbles, photographs, and hand written, original versions of songs and poetry. It is fascinating to see that come to life in a book, collated after the stream of consciousness.
From what has been released so far from the new album Everybody Scream, it looks as though that will be an album that will entirely and utterly break the fourth wall in every single way. It appears to be a deeply personal project. But she has certainly hinted at smashing down the fourth wall in the past.
Excerpt from The Bomb by Florence and the Machine:
I've blown apart my life for you And bodies hit the floor for you And break me, shake me, devastate mе Come here, baby, tell me that I'm wrong I don't love you, I just love the bomb (oh-oh-oh) I let it burn, but it just had to be done (oh-oh-oh) And I'm in ruins, but is it what I wanted all along? Sometimes you get the girl, sometimes you get a song**
From My Love:
I was always able to write my way out The song always made sense to me Now I find when I look down Every page is empty**
And finally, from The End of Love:
I feel nervous in a way that can't be named I dreamt last night of a sign that read "The end of love" And I remember thinking Even in my dreaming It was a good line for a song**
These lyrics take the first person perspective, and are woven into the creative process - but that is not the only way in which the fourth wall lends extra strength to a piece of music. Sometimes, a song will use the second person "you", to tell a story; and that forces the viewer into the shoes of the performer. It is powerful.
What are your favourite songs that break the fourth wall?