I wish they'd keep the next Doctor a secret until he regenerates
That's a bit difficult when they have to spend several months filming in the streets of Cardiff, well before the new Doctor actually appears on screen, and there are Twitter feeds dedicated to finding and publicising locations.
[ DON'T CLICK UNLESS YOU WANT SPOILERS : mobile.twitter.com/drwhofilming?lang=en ]
Steven Moffat has said they do try to keep stuff secret "but we're rubbish at it", so they may as well make the best of a bad job and get publicity out of the announcement.
As for the 'four knocks', at the end of Planet of the Dead (the one with the bus in the desert) the psychic Carmen tells the Doctor 'your song is ending' and 'he will knock four times.' This was widely assumed to refer to the Master's four-beat rhythm that he had used to hypnotise the population of Earth and disguise his presence from the Doctor during the 18 months he was on Earth before becoming PM. In The End of Time, Ten's last story, it is revealed that the rhythm was retroactively implanted in the Master's brain when he was a child, as a link which together with the White Point Star diamond will allow Gallifrey to escape from the Time War. So the assumption was that it was the Master who would kill the Doctor and force his regeneration. In fact, of course, it was Wilf, trapped in the nuclear control booth and tapping four times on the glass.
The Master's craziness was partly the effect of the constant sound of drums in his head since childhood, but also the result of the interrupted resurrection process, which was sabotaged by Lucy Saxon.
And apparently the four-beat rhythm was inspired, not by the beginning of the theme tune, which was a popular fan theory, but by the sound of Russell T Davies' alarm clock...