Uhm, what else would it be? 4 Noctises? Read the OP:
Following the release of the character episodes, Comradesan expansion pack for FINAL FANTASY XVwill introduce a brand-new online co-op mode for up to four players which features intense battles allowing players to take full control of Noctis, Gladio, Prompto and Ignis as a group. Comrades will be available for separate purchase, and is also included in the Season Pass.
And I'm pretty sure it won't be possible to play the main game co-op because they all apparently have their own gameplay (Gladio more Dark Souls-like, Prompto TPS-style etc.), so it will just be its own episode too. It says "which features intenste battles...", which sounds more like a battle arena thing or Horde Mode.