The modders explain that they 'intend to open and recreate some of Fallout New Vegas's map as a new explorable location as a addition to Fallout 4', a hub world sandbox sorta thing. But to manage your expectations: it is not an attempt to recreate Obsidian Entertainment's game Fallout: New Vegas inside Fallout 4.
A new mod named Project Mojave is attempting to recreate a lot of the post-apocalyptic Sin City and surrounding areas as a Fallout 4 mod, and you can play the first slice now. What happens in New Vegas stays in New Vegas, they say, though New Vegas itself will not stay in Fallout: New Vegas.
