4 billion is a database/programming limit, but MC would likely be extremely slow with that many tracks. I think the largest real DB I remember seeing in this forum was about 20 million and it was reportedly fine to use, which is also impressive.
I make that an average of around 87,400 tracks would need to have been recorded every single day in order to reach that number.
I would bet we're long past that. Youtube alone is clocking 300 hours of new video uploaded every *minute*, that's
72000 2.6 million 10-minute clips per day! Granted, it's video not audio, but that only makes it more impressive in my book. The thing is there are billions of people on the planet and half of them now have the means to mass produce and distribute media.
If we're talking *quality* tracks on the other hand...
Edit to fix my math. Stats source:
https://residencelord.com/youtube-statistics/