Here's a review of a 4× adapter (which apparently you can still buy). I also know that there's a specialty tandem drivetrain, Davinci, that uses an interesting 4× setup.
I've never heard of any of the major component manufacturers putting out 4× drivetrains, even as prototypes, and I think that's because it would always be a niche product. Many naive riders are flummoxed by two chainrings, and don't understand how to shift through a crossover pattern correctly (never mind half-step). Adding sprockets in the back makes it easier to shift through a wide range of gears with reasonably close and consistent steps. Also, after you've added enough sprockets in back, it becomes reasonable to remove chainrings in front (my 2×9 bike has 12 distinctly different gears, which could now be achieved with a 1× setup).
Conversely, a 4× setup is only going to appeal to a rider who A) needs an extremely wide range of gears, and will not put up with wide steps between gears, and B) is willing to make the mental effort to avoid cross-chaining situations.
Part of the problem with 4× as a commercially viable product is that it would need to work as part of a system; in order to really benefit from the potential gearing range, the rear derailleur would need to be able to wrap up a huge amount of chain to maintain adequate tension—no major component maker would release a drivetrain that relies on riders intentionally avoiding cross-chaining. Also, the effects of cross-chaining would be exacerbated, and riding small-small could easily result in the chain dragging across the big chainring (The DaVinci avoids this by having smaller "chainrings" that are farther from the rear hub, so the chain's angle of deflection is reduced).
Although I doubt we'll see it happen, the advent of electronic shifting that figures out the right gear combinations for you could make different gearing—such as half-step gearing or 4× gearing—more appealing to a wider range of riders.