Fourth Doctor

The Fourth Doctor is an iteration of the Doctor.

Biography

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

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. (He Jests at Scars... (audio story))

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.(He Jests at Scars... (audio story))