The fourth dimension was equated with time.
The fourth and fifth dimensions were the two dimensions of time in the universe. (PROSE: The Quantum Archangel) Time travel was once described as "pass[ing] through the fourth dimension." (TV: The Ark) Matter accelerated beyond the speed of light would travel in the fourth dimension. (TV: The Claws of Axos) Both Susan Foreman and Ian Chesterton equated the fourth dimension with time. The First Doctor described himself and Susan as "wanderers in the fourth dimension." (TV: "An Unearthly Child") After the TARDIS jumped a time track, the Doctor claimed that he and his companions were in the fourth dimension, which he had always found extremely difficult to solve. (TV: The Space Museum)
After being transported to Mira through molecular dissemination, the Doctor explained the process to his companions as their having been dematerialised, transported through the fourth dimension, and rematerialising at their destination. (TV: The Daleks' Master Plan)