Now that Fallout 4 is here we can begin to learn how to mod it. I've already began to explore the game files and it would appear that the majority of it is extremely similar to Skyrim. Currently there is only a BS2 extractor available from the SKSE team. You can find it via the link below:
Unfortunately at this time the only thing we can seem to really do is modify the texture files. I'm waiting around right now for the Nifskope team to update Nifskope so that it can be used with the Nif files. Once we can open the Fallout 4 nif files then it's just a matter of someone releasing either a 3ds Max or Maya import plugin.
It's only a matter of time now folks!