CE3 is already done and they've been using it on ES6 for a few years now. Starfield was 2 and FO4 was CE1. I doubt they'd hinder the game by going that far back. The Todd said the new one is really speeding up the process, and we know Asha Sharma wants this stuff as fast as possible. Now, Todd might want to keep it close to the vest, and not give Obsidian the newest toys, but it might not be up to him. And I really hope it isnt. But the furthest back I could see it going is 2. which looks amazing in still frames but soulless in movement, and once again, awful at face movement.

So at worst it would be 2 but I really think it may be CE3. Microsoft wants the money and it'd be harder for a lot of people to justify buying a new game with a 4+ year old engine let alone 15+ years.