I think with the new rules, only diehard, every week, lots of TATL or TPAC type flyers can ever achieve 4MM. Certainly doable if you start traveling heavily at a young enough age.
But as someone who started flying DL only 2 years ago, and about to hit 0.5MM, I am quite sure I will end up divorced if I keep up this heavy of a travel schedule for the 14 more years it would take to hit 4MM at my current pace.