http://www.blogtalkradio.com/kotaku/2010/01/06/chatting-with-fallout-and-oblivions-todd-howard
9:30
"As far as the game engine we use, you know it's mostly our own, we started a long time [ago] with the Gamebryo engine, which is mostly a renderer, and that was over 10 years ago now, and over the course of that time we've done a lot of our own development, all the shading and lighting and most now is homegrown stuff. So that being said every time we do a game we spend a lot of time adding to the engine, tooling parts that we feel don't work right, we have very specific needs for the kinds of games we like to make, their size and how big the world is, and we've sort of been doing that with the big new game we're making since we finished Fallout 3. You know we finished Fallout 3 over a year ago so that whole time has been spent re-doing a large part of our basic technology, and things everybody points out in our games..."
So there you have it, proof that they've only been working on the new game since Fallout 3. This means that they've only been working on it for a little more than a year, and with the 3-4 year development time of Bethesda games, they still have a long way to go.
Didn't I read not so long ago that Beth was adding more people to the team so they could take on multiple big projects at once. With their "NEXT BIG GAME" being worked on for app. 14 mos. who is not to say that we don't hear something at this year's E3(surely they have something to show) and we dont see a release date in Q4 of 2011? They have grown in size and will have the capability to produce on a larger scale.
"2010: The Year We Make Games"