I was just looking on the Dr Who wiki site, and they seem pretty adament there that the 4 knocks thing is refering to the "Midnight Entity" seen at the back end of S4. I think it's more likely to be the return of the Master though. At the end of S3, after burning him, his ring was picked up by someone (also, apparently there are shots with Catherine Tate wearing a similar ring, and there were hints about a connection between her and the Master, so you never know!). Also read that there were rumours that Patrick Steward was approached about playing another renegade Time Lord, "The Monk"!