The Four (The Eleven), as his voice is among the voices heard in the Eleven's head during The Eleven (audio story)
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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)
Biography
Post-mortem
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)