John Cage's 4' 33" is the ultimate triumph of context over content.
Whereas modernism first made the idea that context mattered just as much as content, postmodernism realized that context itself was the entirety of art; consider Equivalent VIII, by Carl Andre, which is a pile of paving stones, or the Turner Prize winning Work No.227: the lights going on and off, which is an empty room, with the lights turning on and off at a regular interval.
For Cage's 4' 33", we reach a plateau, musically, for this sort of context-driven artwork. Consider what is generally meant when one talks of "music": the vibrations of the air created by the performance. Music does not exist in an mp3 file, or on a piece of tape or a slab of vinyl, but in the performance of the work. We don't even speak of sheet music as music, or the instruments that create music, as music; they're all facilitators -- context -- for the music itself -- content.
With presentations of 4' 33", all the context remains. The concert hall, the audience, the performer, the instrument; all of it is still present to perform the piece within. Yet the resulting content is missing. What Cage created was not music, so much, as a comment on the context in which music is created that still puzzles its listeners to this day.