The Four (The Eleven), as his voice is among the voices heard in the Eleven's head during The Eleven (audio story)
Talk about it here.
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The Four was the main alias used by the fourth incarnation of a Renegade Time Lord who suffered from regenerative dissonance.
The Four was described by the Eighth Doctor as "weaker than the Eleven". (AUDIO: Stop the Clock)
Liv described him as "the arrogant one". (AUDIO: Day of the Master)
Biography
Post-mortem
According to the Four, he lost so many braincells when he regenerated into the Five that he had to stop wearing shoelaces. (AUDIO: Dark Universe)
After the Eighth Doctor deactivated the Crucible of Souls, the Four told the Nine that each incarnation after him was a disappointment. (AUDIO: The Crucible of Souls)
Right after the Nine kidnapped Liv and Helen, the Four claimed it was his plan to disguise the Clocksmith's TARDIS as a police box to make it look like the Doctor's.
He explained to River Song that the companions of the Doctor kidnapped by the Nine could not escape because the prisoner cells of the prison were shifted fowards in time. (AUDIO: Companion Piece)
When the Nine met the Eleven, their versions of the Four spent time arguing with each other over the Nine's plan to use their immunity to the Ravenous against the Doctor. (AUDIO: The Odds Against)
When trying to work out the identity of the War Master, Liv believed him to be the Four. (AUDIO: Day of the Master)
Behind the scenes
- The script of Dark Universe describes the Four as a "sophisticated game-player, convinced of his own intellectual superiority".