One of the things that keeps Bethesda's games alive long after release is their extensive modding communities. Games like Fallout 3, Fallout: New Vegas and Skyrim remain relevant to this day thanks to the great work of these modders, who are constantly making new content.
Fallout 4 will support mods too, even bringing them to home consoles as well, but just don't expect there to be solid modding tools on day one. When talking to IGN, VP of Development Todd Howard said that all resources were going into the main game first.
Our entire focus is on finishing the game. 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 loo
He then went on to say that they likely wouldn't be around until into 2016.
You can't fault the reasoning here though. Every resource should be going into living up to fan expectations for this game, which are obviously lofty at this point. Also, I highly doubt, this will stop a ton of mods coming out very soon after launch anyways.