Fourth Universe

The Fourth Universe was a parallel universe whose time-stream did not run parallel with the Doctor's home universe, which was relatively known as the Third Universe. Consequently, the two universes' time tracks crossed. The First Doctor told his granddaughter Susan Foreman that it was the furthest that they had travelled since leaving their home planet. (AUDIO: Quinnis)

The Blitzen fish were native to the Aurora Gloriousia in the Fourth Universe. (AUDIO: Relative Dimensions)

The First Doctor and Susan once reminisced in front of Ian Chesterton and Barbara Wright about their adventure on the planet Quinnis in "the fourth universe". (TV: The Edge of Destruction) Although most accounts agreed that they were referring to the Fourth Galaxy, (PROSE: The Edge of Destruction, The Devil Goblins from Neptune) one account instead showed Quinnis to indeed have existed in the parallel universe known as the Fourth Universe, which Susan and the Doctor had temporarily entered through the aforementioned crossing of time-tracks. (AUDIO: Quinnis)