Like most things in parenting, some children will grasp it totally at 4 weeks, some children will struggle & maybe never have a textbook EASY, which is why the whole "listen to your baby" is more important than the timing of EASY routines. Sometimes as a parent our temperament & our child's temperament work together & it all falls into place, but other times it's a juggling act with ups & downs... but I really don't know anyone who has a perfect routine all the time.
My DS2 had great independent sleep until he got sick at 4mo & then that followed by teething & a few other factors & we had 9 months with struggles, DS1 on the other hand was shocking until about 3mo then sorted it for a while & then he started daycare & he struggled perodically from there because the timing of daycare & travel didn't match his natural routines.