QuattroVelo

The QuattroVelo is a commercial-production velomobile which is notable for using both four wheels and also a highly aerodynamic fairing. The QuattroVelo is manufactured by Velomobiel.nl in the Netherlands, with the first production models delivered in 2016.
Features and Comparison.
Velomobiles are a class of faired cycles. One "speed" class of velomobiles emphasizes fairing aerodynamics for high speed. Most velomobiles in this class use three wheels, and so it is instructive to compare the four-wheel QuattroVelo against three-wheel designs.
"Speed" velomobiles (compared to "cargo" velomobiles) are limited mainly by aerodynamics and rolling drag. Thus, "low aerodynamic drag" has long been a focus for such designs.
Three-wheel designs tend to have better aerodynamics than four-wheel designs:
Beyond aerodynamics, three wheels instead of four has other advantages:
Three wheels have at least three significant disadvantages compared to four:
For the above reasons, most "aerodynamic"-class velomobiles have used a tadpole tricycle configuration, while most four-wheel velomobiles have been used for "transportation"- and "cargo"-class velomobiles.
Development.
In 2009, Miles Kingsbury was racing a tadpole tricycle velomobile, and often lost to a competitor riding a streamliner (which Kingsbury had designed and built). Kingsbury concluded one reason he lost to the streamliner was limited cornering stability of his tricycle. He calculated that it was plausible to build a 4-wheel velomobile which would both have the needed stability and also have good enough aerodynamics to be competitive. He then designed and built a 4-wheel velomobile, which he called the Quattro 
Kingsbury finished just in time to ship it to the USA to participate in ROAM, a group of several dozen velomobie riders riding from coast to coast. Several ROAM riders were impressed that Kingsbury in his Quattro had good cornering stability; was able to keep pace with riders with "aerodynaic"-class velomobiles; and had much larger stowage volume—an issue of practical value for velomobiles used for everyday transportation.
This led several velomobile designers to re-consider 4 wheels for aerodynamic velomobiles—despite the inherent drag losses of 4 wheels compared to three. Velomobiles.nl claims for a given width, four wheels offer 40% higher cornering stability than a tadpole tricycle of similar width and center of gravity. Or, a tricycle would need to be 40% wider to get the same stability; but since air drag is directly related to frontal area, making the tricycle wide would directly hurt aerodynamics.
As of 2017/05 there are several projects by various makers to introduce "aerodynamic"-class 4-wheel velomobiels; the QuattroVelo is of special interest because (as of 2017/05) tens of units are in the hands of private owners and so can be used for independent testing.
Comparison.
The QuattroVelo may be compared against other "aerodynamic"-class velomobiles. As of 2017, some of the fastest tadpole velomobiles are the DF, Milan, and Quest. The Quest is made by Velomobiels.nl, but the others are not. Production of the QuattroVelo is (2017/05) tens of units, limiting the volume of independent tests, but several riders have both a QuattroVelo and another "aerodynamic"-class velomobie; and report in public forums QuattroVelo speed is nearly as fast as several "best in class" three-wheel velomobiles such as the DF and Milan.
Another comparison is cargo capacity. One practical use is commuting and/or shopping. A tricycle with a single rear wheel typically has two small compartments, one on either side of the rear wheel. A quadracycle can have one large stowage area between the rear wheels. A single large stowage area is often more useful, as it can hold cargo which cannot be easily divided among several small compartments.
A humorous competition demonstrated that a Quattrovelo can hold 26 six-packs of beer in glass bottles Although the competition was humorous, it demonstrates the practical stowage capacity of the Quattrovelo is both much larger than most "speed" velos, and also large enough for many daily uses. Another use is carrying a passenger. The Quattrovelo is offered standard with a second seat for a small (non-pedaling) rider.
Most 3-wheel velomobiles drive (power) the single rear wheel, and a four-wheel velomobile with two driven wheels may have better drive traction. One reason is that many tricycles put about 1/3 of total weight on the drive wheel, while many quads put about 1/2 weight on the two drive wheels. Further, the Quattrovelo uses two ratchets, one to drive each rear wheel; thus, if one drive wheel loses traction, the other one continues to drive. Note this is different than driving via a simple differential, in which the loss of traction on one wheel causes the other wheel to stop driving. Several Quattrovelo riders are also riders of 3-wheel velomobiles and report online the Quattrovelo has much better traction and climbing on loose or slippery surfaces. There are two- and even three-wheel drive tricycles; so improved traction is not unique to the Quattrovelo, but it may be unique among "speed"-class production velomobiles.
An alternative to "more/four wheels" is a longer and/or wider three-wheel velomobile. A wider profile hurts aerodynamics, but smoother air-flow can compensate to a degree. At the same time, CFRP is stiff and strong enough that the weight penalty for "larger three-wheeler" is comparable to the added weight for a "smaller four-wheeler". As of 2017/05, a main example of a good-aerodynamics three-wheel velomobile with large cargo capacity is the Milan 4.2 ("for two"). The Milan 4.2 is not directly comparable to the Quattrovelo, as the 4.2 has enough stowage for an adult passenger (though no passenger drivetrain). The weight and aerodynamics of the Milan 4.2 are thus broadly similar to the Quattrovelo; and stability depends significantly on cargo loading and weight distribution.