An article on Bethesda.net has details on how Fallout 4 will be supported, outlining plans for regular free updates, DLC, and modification support (thanks Batman). Their roadmap includes a $30.00 season pass that will grant all the DLC Bethesda will release for their upcoming RPG sequel and their mod support will include the release (next year) of the Creation Kit for PC, which they say is the same tool they use themselves. Here's word:

Regular Updates
We'll be doing regular free updates to the game, and like Skyrim, these will come with not just fixes, but new features. For Skyrim we added things like mounted combat, legendary mode, kill cams, visual enhancements and more. We’ll work with all of you to figure out what new things you’d love to see added to Fallout 4, whether they are small tweaks or new features.

Mods & Creation Kit
The most important thing in our games is the freedom you have to create your own experience, and modding is a big part of that. Like all our previous games, we’ve made sure to keep Fallout 4 open and moddable at every step in development. Early next year we’ll release for free the new Creation Kit for the PC. This is the same tool we use in the studio. You'll be able to create your own mods and share them with others. We’re especially excited these same mods will then be coming to Xbox One, and then PlayStation 4.

Downloadable Content
We've always done a lot of DLC for our games. We love making them and you always ask us for more. To reward our most loyal fans, this time we’ll be offering a Season Pass that will get you all of the Fallout 4 DLC we ever do for just $30. Since we’re still hard at work on the game, we don’t know what the actual DLC will be yet, but it will start coming early next year. Based on what we did for Oblivion, Fallout 3, and Skyrim, we know that it will be worth at least $40, and if we do more, you'll get it all with the Season Pass.