My composition teacher told me that he was designated to look after Cage when he came to Liverpool some time in the 70s or 80s. Anyway, he said that he asked Cage about 4'33" and whether it was supposed to be some kind of philosophical statement about environmental sound, forcing the audience to listen to itself. Apparently, Cage's reply was something along the lines of "hmm, yeah, that sounds interesting!", almost like it hadn't occurred to him, or he was just reluctant to be forced to define the meaning of it as something specific.
After reading Silence (probably one of the most tedious but interesting books I've ever had to read), I'd say that seems about right. I think he wants you to draw your own conclusions about the meaning of his compositions, whatever they may be.