Because that's the number conferences that are generally considered to be better than the rest. Just like the "top 4" computer science schools in the US are unambiguously Stanford, UC Berkeley, MIT, and Carnegie Mellon. You can ask, why not 5? Because then you start getting into questions about whether you want to include UCLA or UIUC or Caltech and it's significantly more complicated.
Of course, but there are still four schools which are clearly the "top" ones. The same is true for academic conferences, or big tech, or intelligence agencies.
Top is just marketing. In Big Tech it's market cap or something, but it's not proof of anything and may be just marketing. Google is a search advertising monopoly pretending to be a "tech" company (per Thiel), but is a "top" company to work for. Ok.
And intelligence agencies are government mandated, not marketing made. Or at least I haven't heard any marketing from the NSA saying how selective they are in admissions (as if that means anything).
