We don't know get - no doubt the remaining 3 episodes of Tennant's era (dates to be announced - first one is called "The Waters of Mars") will reveal this.
I would say this reveals the return of the Master as well as the Demon creature in season 2
This is most likely a reference to the Master, whose four-beat knock foreshadowed his return and accompanied him in Series 3. It has been revealed that John Simm will be returning for the last two 2009 Doctor Who specials. Also, when Carmen says the "your song is ending" thing, the music from the episode Midnight is playing in the background. The unknown creature on the planet Midnight knocked four times on the side of the shuttle - this is probably just more foreshadowing of the Master. During The Waters of Mars trailer for the next Doctor Who episode, we also witness four knocks on the side of the base. The Doctor's song ending means that the Tenth Doctor is nearing the end of his life (we know he will regenerate at the end of the last 2009 special or the start of Series 5 in 2010). The first reference to the Doctor's song ending is by Ood Sigma in Planet of the Ood.