Usually short phrases aren’t supposed to be protectable under copyright. However, when a defendant blatantly appropriates a well-known literary phrase for a commercial purpose like selling unlicensed merchandise, courts may make an exception.
[there actually are musicians that specialise in recording backing tracks intended to resemble a particular popular recording which aren't that recording for use in commercial products, but they tend not to get sued...]
Related: The Supreme Court ruled that excerpting a single paragraph from a 454-page book can be copyright infringement. The book was Gerald Ford's memoirs, the one paragraph was his reasoning for pardoning Nixon. The Court's reasoning was more-or-less that nobody cared about anything else Ford did, so excerpting the one paragraph was as good as giving the whole book away for free.
Whether a work in question is even eligible for protection at all is a distinct legal question.
The question in play is how much of of the copyrighted work may be reproduced before it is infringement.
Of course not every set of four notes or eleven letters in a copyrighted work is prohibited; the significance and context of the use matters.
In the latter case, there's also no real risk of accidentally stomping on that.
The claim in this particular case is that they really have generated the entire possible melody space. Legally I think it's likely to fail on multiple levels if it is ever challenged, but part of the point is that some of those failures should also be applied to some real copyright suits that have been won.
(It is somewhat ironic that the music industry continues to be so upset about copyright even as they appear to be converging on The One True Pop Song at speed. Maybe if they acted less like some sort of bizarrely over-trained AI and cranked up the exploration constant, they'd stomp on each other less.)
It's not just the music industry. The whole economy starts to feel like overfitting the profit function.
That's an incorrect way to view it actually. 4'33" copyrightable essence is actually represented by the active production of its scoring. I.e. nothing. The background sound is not what makes it copyrightable. You can go ahead and sit at a piano for the length of the composition all you want, wherever you want, and you'll still be publically performing 4'33".
The rather humorous outcome, if one asks me, is that anyone who writes in 4 beats of silemce into a score should be violating copyright if we're going to be consistent.
That kind of thing is completely unenforceable with respect to performances, but in written musical notation, copying the specific notation pattern could be infringement. If you write "4/4, tempo 80, 91-measure rest", that's maybe violating the 4'33" copyright. If you write a score for a full band or orchestra that shows rests in each measure for each instrument, with key changes and tempo changes and such, you're just retelling the same joke in a different way.
Not even a thought experiment: it’s essentially a 4’33”-long ambient acoustic sample. There are plenty of these (though not usually that long) in sample libraries, recording e.g. traffic sounds, or diner conversation, or crickets in a marsh in summer, etc. And those are certainly copyrighted, unable to be used without license.
I would question any legal professional's authoritative standing to even advise on copyright of a work of music if they miscategorize a recording of ambient sound as a performance of a musical scoring consisting entirely of silence. The copyright doesn't apply to the ambient sound, but to the long quiessence of an artist at their instrument.
It demonstrates a complete blindness of the negative space of music, and a positivistic bias that has no place being enshrined in our legal system.