Bethesda was the first—and, as it turned out, the only—developer to take part in Valve's blink-and-you-missed-it experiment with paid mods on Steam. Given how it defended the practice, it's not unreasonable to expect it'll be at the head of the line again, if and when Valve ever decides to give it another shot, and that the next opportunity could well come in the form of Fallout 4.
But for now, that's not in the cards. "How long was the online payment system? 24 Hours? I can only speak for the present time, but currently there are no plans for a payment system," Game Director Todd Howard told German site Spiegel Online. The interview was Google translated from the original German and is thus a little rough, but the sentiment seems fairly straightforward, and Kotaku says it checks out.
It isn't terribly surprising that Bethesda isn't looking to get back on that particular horse, at least right away. As the studio pointed out in April, paid mod sales made up a tiny percentage of its total Steam revenues, even over a weekend during which Skyrim was free. More to the point, Fallout 4 is only five months away; that's exciting—just 145 sleeps until Fallout day!—but also still pretty close to the first failed attempt. Why saddle yourself with that kind of baggage if you don't have to?