Earlier quoted context omitted.
Ooh, nice one. That's an even better example than Beethoven. Even better still: there's a song which consists of 4 minutes and 33 seconds of silence. That's it - silence. "Your latest video upload contains 5 seconds of stunned silence, which has been identified as an extract of 4'33". This extract is copyrighted. Your video has been deleted."
The creative elements of Cage's work are that it's performed as a piece by an orchestra, and presented as a work (eg a specific length, sold as other works are). Your any-[other]-length-of-silence doesn't infringe. If it did then Cage's piece would lack the artistic distinctiveness needed to make it a work for copyright purposes. Just having a urinal doesn't infringe on Duchamp's "Fountain", not even if it's the same…
True, for 4'33'' there is a simple rule that they probably follow - ignore silence :). But for Fountain (if it ever came up) it's hard to imagine that the difference between a protected copy and a non-protected similar image could really be automatically discovered.