Bethesda definitely intends to make a Fallout 4 in the future. For all we know about it, see:
http://fallout.wikia.com/wiki/Fallout_4_FAQ
The "up to Fallout 6" thing is old and no longer applies to the current situation. Bethesda initially licensed the rights to only 3 Fallout games from Interplay, but has since then purchased full rights to the Fallout franchise, so they can make any Fallout games they want.
Ive heard some pvssyr that bethesda plans "not to stop at fallout 3".
One can only wonder what this means. Either more DLCs or a fallout 4 in the future.
It also means Fallout: New Vegas, which is currently in development at Obsidian Entertainment. See:
http://fallout.wikia.com/wiki/Fallout:_New_Vegas_FAQ
Wait. So Bethesda isn't making New Vegas. I hope this doesn't ruin the game when it comes out.
I haven't been excited about a Fallout game as much as about New Vegas since Van Buren was canceled. I'm very happy that Bethesda isn't making New Vegas, and instead outsourced it to some of the original FO1 and FO2 developers at Obsidian.