4 April 1964: broadcast of "Assassin at Peking", seventh episode of the story we now call Marco Polo. The Khan wins the Tardis from the Doctor at backgammon, but releases the Tardis crew and allows Marco Polo to return to Venice when they prevent Tegana's assassination attempt.
4 April 1970: broadcast of third episode of The Ambassadors of Death. The Doctor and Liz realise that the astronauts are absorbing so much radiation that they must be aliens; but Carrington's men are in pursuit.
4 April 2008: broadcast of Exit Wounds, last episode of the second season of Torchwood; last appearance of Naoko Mori as Toshiko and of Burn Gorman as Owen, the one where John Hart comes back and wreaks havoc across Cardiff, with fatal consequences.
ii) date specified in canon
4 April ?2010: setting of Planet of the Dead (2009).