"Four Knocks" was a musical piece composed by Murray Gold.
It was used in The End of Time Part 2, as the Tenth Doctor saves Wilfred Mott from the Gate's control rooms, which are about to flood with radiation. The piece itself captures the Doctor's realisation that saving Wilfred would kill him and the emotional breakdown that follows.
In the Time of The Doctor it is used again, as Clara reunites with the Eleventh Doctor as a frail old man after spending 300 years in Trenzalore.
Outside Doctor Who Edit
It was used in the prologue of TeamFourStar's Dragon Ball Z: Abridged Special - The History of Trunks.