It appears that Elder Scrolls and Fallout are the Flagship products for Bethesda. They have lots of other games, but these seem to bring in the money.
So I would not be surprised if they are working on both. Fallout 4 might be further along, TES VI is likely just a small team doing some initial brainstorming. It would also not surprise me if they have a general location set for TES VI already.
This is pure guesswork of course.
Sheesh, give 'em a break first!
No. I want TES VI and Fallout 4 ASAP!