This is the same company that denied they were working on any expansions for Oblivion two months before they announced the Shivering Isles expansion.
Also 'Soon' in the gaming industry can be a very short time. As little as six months.
I'm expecting to hear about ESV BEFORE FO4. For several reasons...
1)A lot of Bethesda's fans are also Elder Scrolls fans and are waiting for the next game in the series. Bethesda putting FO4 ahead of that would be a mistake. There'll be a lot of angry fans, many of whom bought Fallout 3 because it was from Bethesda and they needed some Beth-ness to occupy them till ESV.
2) Not enough time for FO4 to have been developed. FO3 was only released in 2008, Bethesda reps have appatrently said that their next big project began development during FO3's dev cycle. This precludes it being FO4 as you don't work on the sequel until you know the current one is a success.
3)New Vegas. Having FO4 come out so close to NV runs the chance of an oversaturation of Fallout.
4)Elder Scrolls is and always will be Bethesda's primary series. They created it, they made it and it's what they made their name with. They won't just throw it by the wayside.