Four minutes 33 seconds is 273 seconds, and minus 273C is the absolute zero of temperature. Cage was certainly aware of this, so although on one level the work is 'the ultimate in freedom of musical expression', it is also based on an ultimate limitation which exists in nature. I also suspect that the work's three movements refer to the triple point of water, which is 0C (or 273K on the absolute scale of temperature).
Thus 4'33' is not merely 'playing nothing for four minutes and 33 seconds', but is 273 seconds of 'absolutely zero' - an example of John Cage's wit and erudition.
W. J. NORTH