In Doctor Who what does it mean that he will knock four times?

The obvious answer is the return of the Master because the drumming he heard is four knocks, but David Tennant and Julie Gardner said on the Planet of the Dead commentary is not what we think it is, Julie also said when we figure it out it will break our hearts.

Just as the doctor believes he's safe at the end of 'The End of Time', he hears four knocks. Wilfred Mott, Donna's grandfather, is stuck in a booth that's about to be flooded with lethal radiation. It is he who knocks four times, and the Doctor, being who he has, entered the booth next to it to allow Wilfred to leave. The Doctor receives the fatal dose of radiation, which, after he visits his old companions, causes him to regenerate.