According to TVmaze episode policy, data in a press release or press kit have precedence over the data on the show's official website.
According to BBC's press release, the episode scheduled for 2018-06-05 considered as "Ep 5/12", so it should be episode 5 of series 3, not episode 1 of series 4 as stated on the show's website. Which also makes 7 subsequent episodes also part of series 3, consisting of 12 episodes in total.