Bethesda confirms that it's going to share the PC system requirements for Fallout 4 as we get closer to launch, probably in October. Unfortunately, it can't offer any estimates, at least for now, in terms of hard drive space or other such things.

Fallout 4 was presented to the world earlier this year, as part of Bethesda's special E3 2015 press conference. As part of the show, the studio revealed gameplay videos, screenshots, as well as plenty of details.
Bethesda also confirmed at that time the platforms for Fallout 4, in the form of PC, PS4, and Xbox One. Since then, not much else has been heard about the technical aspects of the game.
Now, Bethesda's Vice President of PR and Marketing, Pete Hines, confirms that more details about the technical side of Fallout 4 are going to appear as we get closer to launch and things get nailed down.
More specifically, hardware and hard drive space requirements will be finalized about a month ahead of launch, meaning sometime in October.
As such, PC, PS4, and Xbox One owners will find out around that time how much space Fallout 4 will need, while PC users in particular are expected to see the minimum and recommended system requirements for the post-apocalyptic RPG.
Fallout 4 is scheduled to launch on November 10, worldwide.