In Waters of Mars, what did the Doctor think the four knocks riddle meant?

Question

In Waters of Mars, the Doctor claims to have worked out what the four knocks riddle:

No, ‘cos someone told me just recently, they said I was going to die. They said “he will knock four times”. And I think I know what that means, and it doesn’t mean right here, right now, ‘cos I don’t hear anyone knocking, do you? (YouTube clip)

But I can’t work out what he’s referring to. A few ideas that I dismissed:

• He’s guessed correctly. But he seems pretty surprised by it when it happens, and it doesn’t seem like something he’d be able to predict.
• The Master is knocking. But at this point the Tenth Doctor still believes the Master to be dead; he doesn’t learn that he’s still alive until The End of Time.

So do we ever find out what he was referring to here?

Answer 1

At the end of The Planet of the Dead, the episode right before The Waters of Mars, the Doctor has this conversation with Carmen, the psychic woman in the episode:

CARMEN: Doctor? You take care now.

DOCTOR: You too. Chops and gravy, lovely.

CARMEN: No, but you be careful. Because your song is ending, sir.

DOCTOR: What do you mean?

CARMEN: It is returning. It is returning through the dark. And then, Doctor? Oh, but then he will knock four times.

When the Doctor says he thinks he knows what it means, I don't think he's referring to it being the Master, but rather that he can't die until he hears someone knock four times. So as long as he doesn't hear someone knocking, he's safe from danger.