It's hardly a Bethesda RPG if you can't replace some part of it with the chugging terror of Thomas the Tank Engine, but Fallout 4's mod tools won't be ready at the game's launch on November 10th. Speaking to IGN, Bethesda's VP of Marketing Pete Hines said that, their "entire focus is on finishing the game."
"Nobody cares about mods if the game sucks," said Hines. "This has always been our philosophy. We shift energy to construction tools, the creation kit, and all of that stuff once the game is done, and we start to figure out what all of that is going to look like."
Modding tools have long been a component of Bethesda's games, but the tools for Skyrim, Fallout 3 and Oblivion all came post-release as well. In the case of Fallout 4 [official site], the situation is likely complicated by the tools also being designed to work on the Xbone.
"The idea is that [the tools] will work on all platforms, but the truth is the system doesn’t exist," continued Hines. "It’s still being built and worked on and it’s going to take awhile. It’s going to take clearly into next year because we can’t even start it..."
Bethesda games tend to be perfect for modding not only because they release a robust toolset for making them, but because their open world's are usually easily tilted or twisted towards neighbouring genres. It's hard to play Fallout 3 for example, with its meager radiation effects and half-hearted need to drink water, and not imagine what it would be like to play upon a harsher, more survival-focused wasteland. Skyrim similarly begs the question of what it would be like if the cold weather was actually dangerous. Mods that those things now exist for both games, and it seems likely that Fallout 4's scope will leave similar dangling threads for modders to run with.
Or, for reals, they'll just add Thomas the Tank Engine.
If you can't wait till next year to start modding Fallout 4, you could always spend your time modding Fallout: New Vegas to look more like Fallout 4.
Otherwise here's Fallout 4's E3 presentation in full, which includes detail of its new building and crafting system: