User blog comment:DeathGodPrayer/Talk About What You Want!/@comment-1359322-20140119033603

So the themes for the 4th story arc are "Generation Change" and "Brothers".

The "Generation Change" probably refers to Rias and Akeno graduating, while Issei and others taking over as 3rd year students. (Ishibumi never specified it).

And "Brothers" refer to Sirzechs' brother-in-laws: Euclid and Issei.