The 'he shall knock four times' refers to the Master I think because when the Master drums is fingers to show the Doctor what the drums in his head sound like, he knocks four times. If you speed up the sound of the knocking when watching the episode it sounds the same. But then again there is a rumour that the Rani is going to make a comeback at some point so it may be her. But the most likely candidate is the Master.

I too reckon it'll be the Master becos I think his wife picked the ring up or sumthing, so she mite bring him back to life.

But cud someone please answer my question?
My question is put your answer on a comment please): Why have some of the answers been captain Jack? Because Jack isn't a villain, he's a hero. And he's the Doctor's friend. AND he's the face of Boe, so why is he on the list?

i think it could be Rassilon(most likely played be timothy dalton), but the master is definitely going to be back.

i thin it's the master because in the last episode with him someone picks up his ring. Or it could be girlfriend because the fingers are manicured just like hers. I dunno who it is, but I betcha it'll be exciting.

As incredible as it seems, the Master!