Damned if I have a guess as to what the "knock four times" clue is about — care to enlighten me? (At least I can be almost sure it's not the fucking Master, a villain I've never liked, what with there being only two more episodes to go this year.)

[spoiler] It's almost certainly the Master, from what I heard. The knock four times is the sound of the drums. [/spoiler]

This makes me unreasonably happy, though I don't think it'll necessarily be in the next two episodes.