Not sure if anyone else plays Blood Angels (Panzer?), but I picked up my copy of the codex today. In the section chronicling what happens after the battle at Baal, the book refers to Blood Angels and Flesh Tearers halting a 'Tau colonisation fleet' which was transported to the Red Scar by the warp. I'm guessing this is at least part of the missing fourth sphere of expansion?
It doesn't say much else, only that the fight continues. Maybe we'll see a Blood Angel vs Tau campaign at some point. Or it just gives us a narrative for the two armies to fight it out.