I was inspired by that other thread kkkk. So, generation just mean the time you debuted and end! We had cycles and the common thing was: when the oldest ones disbanded or were overtaken then the other gen started officially, but now kpop is at a phase where disbanding isn't the normal anymore.
We arrived at the true value of groups outliving the previous one and just outliving themselves with renewing to more 3/5 years so I think that generation now is just referring to a time of debut.
I don't see BTS going anywhere of disbanding neither the others top groups tbh. So 4th gen started and end kkkk.