A group of Fallout modders called Road To Liberty have uploaded a video to the Wonderside YouTube channel showing gameplay of their creation of Capital Wasteland from Fallout 3 playable in Fallout 4.
The area will be fully furbished, complete with quests, dialogue, locations, environments and more. What should also be recognized is the team is also going to be using Fallout 4's Creation Engine to build the same weather effects and atmosphere that was felt in the original Fallout 3.
Road To Liberty very well could pull this off and from the gameplay video found above it seems that they have been doing a great job so far. Hopefully in the near future we will hear more information about how the mod is coming along or if it is even realistic that it will leave alpha.