https://www.wikiwand.com/en/Doctor_Who_(season_4)

The fourth season of British science fiction television series Doctor Who began on 10 September 1966 with the 1st Doctor (William Hartnell) story The Smugglers and, after a change of lead actor (Patrick Troughton) part-way through the series, ended on 1 July 1967 with The Evil of the Daleks. For the first time, the entire main cast changed over the course of a single season (the only other occasion this has happened is during Season 21). Only 10 out of 43 episodes survive in the BBC archives; 33 remain missing. No serials in this season exist in their entirety