A Level 4 planet was a designation used by Galactic Law in reference to certain worlds, with the implication that it referred to relatively primitive planets not yet fully integrated into galactic civilisation. Other designations included Levels 1, 2, 5 and 6.
The Jariden Colonel Ulrik described 19th century Earth as possessing sub-Level 4 technology. (AUDIO: The Four Doctors)
Romana I once referred to Earth in 2011 as having a level 4 civilisation, (AUDIO: Ferril's Folly) despite it being designated a level 2 planet in the 1940s, (PROSE: Ghosts of India) and having a level 5 civilisation from at least 1957 (PROSE: First Frontier) to 1979, (TV: City of Death) 2008, (TV: Voyage of the Damned, Revenge of the Slitheen, The Eleventh Hour) 2009, (TV: Partners in Crime) and into 2040. (AUDIO: Phantoms of the Deep)
River Song, in the spaceship Sarah Jane, once lured a SporeShip away from a inhabited level 4 world. (AUDIO: The Rulers of the Universe)
The Quiet One's home planet was a level 4 society. (AUDIO: Shadow of Death)