The Smart Forfour was a supermini that was produced by Smart between April 2004 and June 2006. Unlike the other models of the marque, the Forfour was a more conventional five-door hatchback with a relatively roomy interior, available as a four- / five-seater.

The car was produced at the NedCar factory in the Netherlands in conjunction with Mitsubishi Motors. This is the same factory that produced Volvo 300 cars in 1970s and 1980s and the Volvo v40's in the 1990s. To save production costs, the Smart Forfour shares most of its components with the 2003 Mitsubishi Colt. This includes the chassis, suspension and a new generation of MIVEC petrol engines, ranging from the three cylinder 1.1 to the four cylinder 1.3 and 1.5L with power up to 80kW (109PS). The 1.5L cdi diesel engine, on the contrary, is a three-cylinder Mercedes-Benz engine derived from the four-cylinder of the Mercedes-Benz A-Class, and is available with either 68PS (67hp/50kW) or 95PS (94hp/70kW).

A Brabus-prepared sports version has been available since 2005. It is powered by a turbocharged Mitsubishi 4G15 engine, developing 130kW (177PS), 27PS (27hp/20kW) more than the Mitsubishi Colt CZT. It can reach a maximum speed of 221km/h (137 mph) and accelerate up to 100km/h (60 mph) in 6.9 seconds. The Forfour was to be sold in the United States along with the revived Smart Formore vehicle. However, due to slow sales, the Forfour has been phased out from production.

The sales brochures state that the interior "is designed around the concept of a lounge"; to test this, Top Gear presenters spent 24 hours inside the Forfour. They said it was a nice place to be in but they would not buy the car due to its relatively high price compared to its rivals.

Following Smart's success launching the fortwo in the US, and due to a surprise increase in popularity in the forfour, Mercedes-Benz exec Rainer Schmückle recently revealed that officials were considering relaunching the car.

Main article: Plug-in hybrid

A Smart forFour has been converted into a plug-in hybrid by Lithium Technology Corporation and Zytek Systems. The lithium-ion battery can propel the vehicle up to 84mph (135km/h) and last on its own for up to 20miles (32km). The engine is a combination of a 68 horsepower, 1.5 liter, 3-cylinder turbo charged diesel engine and two high-efficiency permanent-magnet electric motors. It was awarded by the Energy Saving Trust for the “Ultra Low Carbon Car Challenge” project .