Actually, it is not so clear, while I have seen the statement in various news articles like
this one that Starfield has been in full production since Fallout 4 was finished, the
interview these are based on says only that they did not actually start development on the game until then.
Another interview from March 2018 however stated that at the time there was one project in full production using the old animation system (now we know this probably refers to Fallout 76), while a new animation system was being finished for a project in pre-production, which seems likely to be Starfield. That suggests the status changed from pre- to full production sometime during Q2 2018, but again, it is not clear exactly how much work was done until then, obviously it was more than just concept art if it involved engine development, and there was some kind of playable demo (even if only a vertical slice) by June.
I re-wrote this post like 6 times as I learned more and did some research.
“We just talked about it, and then I guess it really picked up steam — we’re going to do this, register the trademark, about five years ago,” Howard told
VentureBeat. “Then we would talk about it from time to time during that period. What are we going to do? And we started work right after
Fallout 4 was finished.”
You aren't wrong. I was so ready to say you were, but after looking into it, as you said, it isn't entirely clear what they mean by "started work." Really goes to show how much the media can misrepresent stuff, even unintentionally!
It just seems so unlikely to me that full production didn't start until 2018. But I guess it's possible.
Looking back at their timeline, we have...
Morrowind in 2002, Oblivion in 2006, Fallout 3 in 2008 (only 2 year difference! for such a big game!), Skyrim in 2011, Fallout 4 in 2015...
I don't consider 76 or New Vegas main entries, but more spin-offs, so I omitted them from the list here. Worth noting that Bethesda likely has multiple studios working on various projects.
I'm not sure what conclusions to draw here, then, other than that there really
is no conclusion to be drawn.
I still think Starfield is a 2020 game. I doubt they are going to let fans sit on the disaster that is Fallout 76 for another 3-4 years.