There is a common myth that the length of 4'33" was a deliberate choice. Since the piece is silent, some believe that it was selected because the number of seconds (273) corresponds to absolute zero (which is -273.16º celcius). However, John Cage selected the lengths of the movements via chance operations, as he did many of his pieces. In this particular case, as with many of his compositions of this era, he used the I Ching as a basis for generating random numbers for use in his composition. This is fairly well documented, especially in one of the few biographies of Cage, "The Roaring Silence".
It should be noted, that with pauses between the movements (as it is sometimes performed), the time is actually longer than the title would indicate. 273 seconds is a neat coincidence, and nothing more.