I agree with the remarks of several favorable reviewers here.
For much that I've always loved this show, this premiere astonished me with a fantastic twist in the narrative and lifted the memory left by the third season's finale.
The complexities of the "Possible Conjectural in Politics" are shown through the lenses of an alternate reality (a very clever narrative choice) but with a nuanced and smart turn.
Diane slides from the immediate, emotional, almost childish elation of the political "fanactivist", to a more reflected assessment of the contradictions and costs involved in the protection of the greater good. The dramatic transition from her embrace of the alternate reality to the suspicion of being into a dream, to her waking up marks a high point in the episode!
Old political dilemmas shown through a masterclass of screenwriting and direction with a permanent wink to the viewers' intelligence. I love this show and I celebrate that they have raised the bar with this new season!