The 2nd generation changed the game for 3rd generation and the 3rd generation made kpop much more popular overseas. Especially BTS, Blackpink's and Twice's international success changed the game. And I don't say it's because of them that 4th generation groups are selling so much but that they are part of it and that they brought kpop international on the next level. Their fame has definitely brought more fans to kpop.
Of course there were other groups like EXO or Red Velvet etc., but in my opinion these three are mainly responsible for it.
And just like the 2nd generation paved the way for the 3rd generation, the 3rd generation paved the way for the 4th generation. How much the fourth generation will pave the way for the 5th generation, we will only see when the fifth generation starts.