That seems to be the trend, at least, for the most part. After Oblivion, the team spent their time on Fallout 3. After Fallout 3, they are likely working on TESV, and Todd Howard has already stated that they have numerous ideas for Fallout 4. While Beth works on their current project, Obsidian has been working on Fallout New Vegas, a spinoff from Fallout 3's vision of Fallout. With Zenimax aquiring Arkane Studios (who have reportedly been working on a Bethesda project for at least a year), do you think that Arkane will be crafting an Elder Scrolls Spinoff? Remember, Arkane is the studio that made the award winning Dark Messiah of Might and Magic (the only competition on the market against TES Oblivion when it was released).
My Take: Arkane is likely working on an Elder Scrolls spinoff, given their history with first person high fantasy games, and their work on Bioshock 2 (which they were 1 of 4 supplement developers). This would also give Beth time to develope Fallout 4, while Arkane works on an Elder Scrolls spinoff like Obsidian is with New Vegas. This would give us 4 open world Betheda published/developed RPGs over 6 or so years...far more than any other developer/publisher would be capable of putting out.