Fourth Doctor

The Fourth Doctor is an iteration of the Doctor.

Biography
The Fourth Doctor was sent back in time to stop the Daleks' creation, but failed. (TV: Genesis of the Daleks) He later went to Logopolis, which was destroyed around the time of his visit. The Doctor was believed by someone to have destroyed it (AUDIO: He Jests at Scars...). He would've later regenerated into the Fifth Doctor. (TV: Logopolis)

The Valeyard's "Course Corrections"
When the Valeyard was fixing a few of his past mistakes, he went back in time and gave the Thals a formula to destroy the Daleks before their creation. He later decided to stop his fourth self's visit to Logopolis. (AUDIO: He Jests at Scars...)

The Valeyard's Accidental Interference
When he was trying to stop his younger self's visit to Logopolis, he accidently destroyed his earlier self's TARDIS by accidently time-ramming it, along with also time-ramming the Master's TARDIS. (AUDIO: He Jests at Scars...)

Additional or Possible Event Erased
Either at a point before, during, or after the events of Logopolis, the Fourth Doctor did, or would’ve met, the Valeyard. (AUDIO: He Jests at Scars...)

Possible Death or Possible Further Erasure
The Valeyard told Melanie Bush that he destroyed any possible chance of himself existing. It’s possible that the Fourth Doctor was killed or erased at a point before the events of his trip to Logopolis. (AUDIO: He Jests at Scars...)