I'll answer it as what makes it the most playable, I'd not be surprised if there's a positive feedback loop from a bunch of 4K VSRGs and even outside them. Specifically with the amount of lanes.
Of course, you have games like RoBeats and Friday Night Funkin' with mainly 4k gameplay. But those games could have been 6k or 7k or whatever, so something inspired them to be 4k, and I'm not doing the research to find out where it all started! My best guess is probably DDR. But it's no surprise those games eventually encourage the playing of this game in 4k at first.
Mobile rhythm games do have a tendency though to split their lanes into 4, even if they can have more (like PJSK) or have a unique mechanic (like Arcaea's sky notes or the simple shifting of the notes in Phigros). Those games tend to encourage the usage of one or two finger of each hand tapping two different "lanes" each.
And then there are games like Sound Voltex which is 6k + knobs, but it presents itself as 4 lanes as far as I can tell.
I feel like there's a bunch more examples I don't care enough about to know, but those are a few. Just do not mix this up with me saying "Oh playing PJSK is the same as playing osu!mania", or sound voltex is the same, etc.
edit: If I am right about it being the amount of lanes, people must really like looking at their musical staff. Even it has 4 lanes lol!