I thought I would comment, before anyone else does, that the argument in this posting is among the most sloppy intellectually that you'll find on this site (feel free to add your other nominations!). The conclusions about longer copyright periods do not 'solve' the problem of recompensing John Cage during his lifetime and freeing him to do more of the work he wanted to do. And, anyway, it is a bit daft to pretend that you can create more or better Cages through any kind of legislation. That would be like trying to make better honey by changing the speed limit.
I now have my own copy of the sheet music for 4'33". I paid £4.95 for the 'original version in proportional notation' — this is different from the version that was originally published, if you follow me. The latter is available for £4.33 (a bit gimmicky if you ask me) from the Peters Edition web site.