The two shows will share an airdate for the first time on October 1, with Doctor Who transmitting first.
'The Wedding of River Song' - starring Matt Smith and Alex Kingston - will air at 7.05pm on BBC One.
The Merlin premiere - titled 'The Darkest Hour' - will then follow at 7.50pm. If the series airs uninterrupted, the fantasy drama's fourth run will likely conclude just prior to Christmas.
Stars confirmed for future episodes of Merlin include Whitechapel's Phil Davis, former Spooks star Gemma Jones, Battlestar Galactica's James Callis and Rome's Lindsay Duncan.
Meanwhile, Doctor Who showrunner Steven Moffat has promised that many lingering plot threads will be resolved in the show's finale.
Watch a trailer for the fourth series of Merlin below: