The title of the opening episode to Season 4, "The Beginning of the End" sums up entirely what this season is about.
In my opinion, Season 4 is a bridging season between Season 3, and Season 5. I know right, it obviously is being that it's in between, but what I mean, is that it is there to connect the dots. At the end of Season 3, we see in a Flash Forward Jack wanting to go back to the Island. So, we as an audience needed to understand why.
This season shows us why, and then right at the end, picks up where we were left, with Jack in need of returning.
It truly is the beginning of the end here, and the doors are opened for the penultimate season.