FO4, for all we know, is already under development (or will be shortly once Skyrim is released.)
Also, Bethesda didn't make New Vegas, Obsidian did.
Finally, not many people like DLCs, they think it's an excuse to milk more money from players. But, if they do make another DLC, it better be high-quality, none of that Horse Armor or Mothership Zeta crap.