I don't think Obsidian will be involved given they're working on their kickstarter project and the basic premise of F4 would have been penned already. While I'm going to miss Obsidian's writing and side quests, I'm still excited for F4 especially if it's next gen. I have seen how much they pushed with Gamebryo with Skyrim and it would be interesting to see if they are going to be using a new engine for this game. I hope it does, especially if it's going to be out for next gen consoles. Of course, if it's going to be out for the current gen system as well, then I'd assume it'd still use Gamebryo.
They just spent an extra year of development on Skyrim (3 years instead of the usual 2 and a bit it takes for them to make a game) rewriting/reworking major parts of their engine. Suffice to say, any new Fallout is going to use it.