4 '33" is actually very difficult to perform, since the musician must still keep a proper beat without any aural cues . It's also fascinating to hear, because absent the music from the stage, an audicence makes a surprising amount of noise, even a well-mannered crowd such that might attend a performance of John Cage's music.
However, it is NOT silent. It's 4 and a half minutes of people coughing, rustling their programs, lightly shuffling their feet and perhaps whispering. The point of the piece is to get the listener to hear the world around him or herself as critically as he hears a performance.