There has been a recent bizarre development with this "song". A British composer named Mike Batt became subject to a plagiarism charge after he included a song, A One Minute Silence, on his band -- The Planets' -- classical rock album.
When originally presented with the court case Batt was astonished. He said, "Has the world gone mad? I'm prepared to do time rather than pay out. We are talking as much as £100,000 in copyright."
Despite his initial amazement, on September 23, 2002 Batt settled his case with the John Cage Trust by paying out an undisclosed "six-figure" sum and released these words: "We are ... making this gesture of a payment to the John Cage Trust in recognition of my own personal respect for John Cage and in recognition of his brave and sometimes outrageous approach to artistic experimentation in music."
Ironically Batt's love for John Cage is probably what hurt him the most. The liner notes on Batt's version of the silent song list the songwriting credits as "Batt/Cage" -- in effect admitting he knew that his song was based on the earlier work.
Humorously Batt also remarked, "Mine is a much better silent piece. I have been able to say in one minute what Cage could only say in four minutes and 33 seconds."
Update 6/12/2022. It appears that Mike Batt later claims in a 2010 BBC interview that the entire lawsuit and settlement never really happened and it was all a publicity stunt. See: https://www.bbc.com/news/uk-england-hampshire-11964995
Zerotime says re 4'33": Rob Dougan did a 33-second silence as a track on his Furious Angels album, and I'm pretty sure he hasn't been sued yet.
note: disk 1 - track 13 Pause on Furious Angels is the song mentioned by Zerotime.