Originally Posted by
Val the Moofia Boss
From what the people who have actually played Sen 3 + 4 have been telling me (as well as the asses who PMed me plot spoilers), this is untrue. The Azure-Zero plan isn't relevant to the Phantasmal Blaze plan; all of those plot points stay in Crossbell.
While CS3+4 feature cameos from characters in previous arcs, the Empire arc is at its heart Rean's story. If you're going to play a previous game, I suggest you play FC (mind as well play SC too while you're at it), as CS3+4 very much builds upon the ending theme of that game (FC).