I doubt they've even started it. Skyrim DLC Needs to be worked on
It's more than a couple people working on the game, and it isn't all the same people working on both games. I don't think there is a professional game studio out there that only works on one game at a time. And DLC doesn't need that many people to work on it compared to the full game, hence the mods that come out that are just as good or better than any dlc that comes out.
They've been working on Fallout 4 since Fallout 3 came out. They no doubt have the game all planned out and have gathered much of the preliminary resources.