https://www.hooktube/watch?v=Y7dGgUr4qiU
"This game has more Fallout 4 assets than Fallout 4."
He was joking, but he's actually correct.
If you look at the "new" monsters that they're adding for 76, they're clearly a lower resolution than the rest of the Fallout 4 textures, you can see areas of the monsters that are clearly marked out for tessellation but aren't utilized, the MG-42 is blatantly a lower resolution than the majority of weapon textures (not all fallout 4 weapon texture resolutions are the same, yes they are this shit) and you can also see areas that would be tessellated (fake depth on a 2d plain to save on poly count, time intensive to do but looks better and saves on performance CPU wise) but aren't.
Judging by the lack of properly implemented tessellation on their gameplay reveal, i'm going to make the claim that they haven't been working on fallout 76 for more than 1 and a half years at the most.
They blatantly had the multiplayer in development at the same time they were making Fallout 4, Bethesda is a relatively small company compared to their financial status so i'm going to assume they either had to set up a whole new office (which Zenimax would not do for a single game, they're jews) or they hired the MP out to a third party OR (the worst one) they actually had some of their Fallout 4 team diverted to work on a multiplayer element.
I haven't heard anything about 76; nothing about any peripheral software used for the multiplayer other than baseline networking tools that everyone uses, so it's safe to say that Fallout 4 development time was slashed to ease the load on both 76 and future multiplayer Bethesda games. This actually explains why they chose the "gameplay" elements they did for Fallout 4, every gameplay choice they made required the absolute least amount of polish to make workable. This allowed them to push fallout 4 out as a $60 asset bug test/showcase for the new $60 Fallout 76.
TL;DR I think Zenimax made Bethesda cut their development time on Fallout 4 in order to make the most amount of gameplay assets that could be used in a future more profitable online only fallout game, this was the main goal of Fallout 4 on the corporate level.
Zenimax is going to run Bethesda into the ground within the next decade EA style