Speaking, Bethesda's Pete Hines confirmed that they won't be available from launch, as the team focuses on getting the core game whipped into shape first.
"Our entire focus is on finishing the game," Hines said. "Nobody cares about mods if the game sucks. 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."
The fact that mods are coming to consoles too may delay the release a little, with Hines admitting that functionality is "still being built and worked on and it’s going to take awhile," probably some time into 2016.'s Creation Kit didn't arrive until some three months after the main game, and I'd expect a similar schedule for Fallout 4.
In any case, the main game should keep you busy for a not-inconsiderable amount of time when it launches this November 10.