Ben Aaronovitch (one of the writers from Doctor Who of old..ish) once stated that if he had to finish the Doctor off he'd do Ragnarok.
By the way I think the Master being the one who knocks four times is way too obvious (even though he's in the episode). I'm going to hedge my bets and guess that it's someone else:
Spoiler for Potential Doctor Killer:
I think it's going to be someone out of left field and my suspect is the story's companion - Wilfred Mott (Donna's grandpa). I think the Doctor is going to inadvertently endanger Earth while trying to resurrect Gallifrey... and a teary eyed sorrowful Wilfred is going to do him in to save the world.
I think thje fact that the Doctor confides in Wilfred with regard to his foreknowledge of his imminent death... will add some great dramatic tension as/if it slowly dawns on Wilfred what he has to do.