^ Every source I've seen agrees that the season premiere was multi-centric. We've had a few of those before.
As for the seasons 1 - 3 format with the flashbacks, I suspect that the show will return to that once the Oceanic 6 are back on the island. We still have backstories that can't easily be explored without them, like Desmond's time in prison, Libby's story, and the three years the Oceanic 6 were off the island. I think that a big part of why we don't have regular flashbacks now is because the action is already split up enough between the mainland and crash survivors in different timelines.