Bethesda confirmed at the Microsoft E3 conference that Fallout 4 on Xbox One like PC will allow modding. Mods made for Fallout 4 PC will be compatible on Xbox One.
PC to Xbox One transfer of mods will be free to the user and comes thanks to Windows 10. Though usable on Xbox One, the mods will only be editable on PC.
It was also announced that Fallout 4 buyers on the Xbox One will get Fallout 3 for free, this is most likely taking advantage of the backward compatibility that was just announced.
Happy to hear about Fallout 4 modding? Personally this has really made me consider cancelling my PS4 pre-order and instead get Fallout 4 for Xbox One instead.