Count Fortemps is narrating Heavensward in past tense exactly the same as ARR was narrated by Louisoix(?). It's a common story telling technique and doesn't mean anything for the story itself beyond our knowing that Count Fortemps will obviously survive the events to write his memoirs.

As for 4.0, all we've been told is that it likely won't require completion of Heavensward's MSQ due to the complaints over needing to complete the 2.X story. They were talking about adding a way to just watch an abridged version of the story for players to catch up and then start the new content if they want to. 4.0 won't be a new game or anything like that since 3.0 and the 3.X arc are already tying into 4.0's story now. It'll likely be level 60-70 so new players will still need to level up and play through the old story content to level regardless (since it's the fastest way to level).