The Fourth Reich was the term for hoped-for Nazi empire after the fall of the Third Reich.
Adolf Hitler, Jr. was smuggled out of Berlin by Eva Braun, and was raised to revere his father's memory as the new leader of the Fourth Reich. (PROSE: The Shadow in the Glass)
A neo-Nazi group in 1988 attempted to take control of the Nemesis statue and use it to establish a Fourth Reich, but were foiled by the Seventh Doctor and Ace. (TV: Silver Nemesis)