During pre-production of Fallout 4, Bethesda actually tossed around a number of ideas relating to the possible inclusion of multiplayer mode. According to its developers, adding a multiplayer mode into the game is something they do with each and every one of their titles just to imagine what it would be like, presumably even Skyrim. They talked about that here.
Despite the fact that multiplayer mode won't be making it into Fallout 4, fans have continued to ask Bethesda for its inclusion, seemingly oblivious to the fact that the game won't have it unless someone figures out a way to mod it into the game.
Commenting on tweets to his Twitter account about the addition of a multiplayer mode to Fallout 4, Bethesda's Pete Hines responded with perhaps one of the snarkiest comments we've ever seen coming from an official game developer.
Hilarious.