The Fourth Fleet was a military fleet of spaceships utilised by the Dindrinines and launched from Dindrinous Beta.
Although supposedly powerful, it was once all but wiped out save for Gebra and Hurum, who were rescued by the Thirteenth Doctor and left to be picked up at the Gardens of Everlasting Summer. However, they were missed due to a time storm at the gardens, leaving them to mistakenly believe that the Doctor had abandoned them on purpose. (COMIC: The Everlasting Summer)