Even before Nathan-Turner's day, Robert Holmes had decided that four episodes was the 'ideal' length, hence to the shift during his time as script editor from 4-4-6-6-6 to 4-4-4-4-4-6, with the 6 tending to be 4+2: Seeds of Doom is 2+4, Talons shifts focus from Chang to Greel four episodes in [partly because episodes five and six weren't written when production began!], and the Vardans-to-Sontarans shift four episodes into Invasion of Time was his suggestion too (the original script had fallen through; Holmes was asked to write a replacement. He said 'No' as he'd only just left the show, but did donate some story ideas and the Sontarans).
Even Armageddon Factor the next year shifts focus from Atrios in the first two episodes to Zeos for three and four and the Shadow's domain in five and six. And Shada does something similar, or would have done (Cambridge to space to Shada itself)
Even Armageddon Factor the next year shifts focus from Atrios in the first two episodes to Zeos for three and four and the Shadow's domain in five and six. And Shada does something similar, or would have done (Cambridge to space to Shada itself)