it hasn't started yet.
"4th generation" fans are acting like this so called generation exists in a vacum. In my opinion, you can't call groups leaders of their generation when they are still lagging behind the leaders of the previous generation. what are they leading exactly? and you can't say a new era started when the last one hasn't even started slowing down let alone ended.
let's talk about how the 3rd generation started, in 2013 EXO started having huge hits but not only that, they actually outsold the top groups of the previous generation (in physical sales), no one could argue that Big Bang or Super Junior or TVXQ were still the absolute top group. and then the gap between EXO and other contemporary groups was huge. when Twice debuted, the GG scene was relatively bare, there was RV and they were successful but they weren't a dominating force. Twice then started having huge hits and outsold the previous best selling GG aka SNSD. that's how you become the leader of your generation, when you absolutely outdo the previous top groups, it becomes your era.
And then BTS and Blackpink outdid EXO and Twice respectively in pretty much all metrics, but considering that they debuted around the same time as the latter 2, it is still the same generation.
something else that imo marks the start of a new generation is when the top groups of the previous one start vanishing (enlistment, disbaning)
If we look at the start of the 2nd generation, it was quite similar too. maybe they didn't outdo the 1st generation in physical sales but most 1st generation groups either disbanded or stopped being active.
none of the above is true for the current kpop scene, BTS and Blackpink still have a very strong hold of kpop and they are still way ahead of everyone else. Arguing that we are in a new generation means that BTS and Blackpink's era ended but that's ridiculous, they are literally still outdoing themselves and still going strong. so why would anyone think that a generational shift happened?
and it isn't only BTS and Blackpink, newer groups have yet to match let alone outdo other top groups of the 3rd generation like EXO, Twice, RV, Mamamoo, Seventeen, NCT etc.
the only reason there would be a 4th generation is if it existed in a vaccum and we pretended the actual top groups don't exist.