https://www.pcgamer.com/fallout-76-is-based-on-plans-for-fallout-4-multiplayer/

Bethesda considered adding multiplayer to Fallout 4 but put the idea on hold to prioritize its singleplayer story, Bethesda Game Studios director Todd Howard explained in a new Noclip documentary. However, Bethesda eventually revisited Fallout 4's would-be multiplayer and turned it into Fallout 76, which we now know to be an online survival game.

"Fallout 76 is the multiplayer design from Fallout 4," Howard said. While developing Fallout 4, Bethesda wondered: "Hey, should we do multiplayer? 'Probably not.' What would it be? 'OK, it would be this.' Let's talk it through. 'OK, that's pretty awesome.'"