After Porsche withdrew from Formula One at the end of the 1962 season, the company refocused on sportscar racing. The 904 debuted late in 1963 as a successor to the 718 RSK series. Porsche produced 106 cars at four or five per day, priced at US$7,245 FOB Stuttgart, with orders exceeding the homologation minimum of 100 units. The car's official sales name was the Carrera GTS rather than its internal development code 904, because Peugeot owned rights to the naming convention of three-digit numbers with a zero in the middle — the same reason the Porsche 901 had been renamed the 911.
The 904 was the first Porsche to use a fibreglass body bonded to a steel chassis for additional rigidity, achieving a drag coefficient of 0.34. Its mid-engine layout was inherited from the 718 RSK series. Power came from the 1,966 cc Type 587/3 four-cam flat-four engine producing 180 horsepower, driven through a five-speed transmission. The suspension used coil springs and unequal-length A-arms in front — the first Porsche not to use trailing-arm front and swing-axle rear suspension. Race-prepared four-cylinder cars weighed approximately 655 kg, enabling a 0-60 mph time under six seconds and a top speed of 160 mph (260 km/h).
Some 1965 models used a variant of the 911's flat-six engine in the 904/6 configuration to meet demand. A smaller number of factory race cars received the 225 horsepower 1,962 cc Type 771 flat-eight cylinder engine derived from the 1962 Porsche 804 Formula One car, in the 904/8 configuration. The Type 771 engine was noted for a tendency to shatter its flywheels, limiting its broader application.
The 904 made a tentative debut at Sebring in 1964 with clutch trouble, but immediately recovered at the Targa Florio, where a four-cylinder 904 took an outright overall victory. The car then placed third at the Nürburgring and all five factory starters finished at Le Mans, placing in the top twelve overall against much more powerful competitors. A customer 904 driven to the Reims circuit fresh from Stuttgart went on to win the race without requiring any spare parts.
For the 1964 season, the 904 scored a 1-2 at the Targa Florio and class wins at Spa, Sebring, the Nürburgring, Le Mans, Watkins Glen, Zandvoort, Canada, and the Paris 1000 Kilometres, also winning rally events including the Tulip, Munich-Vienna-Budapest, Geneva, and Alpine Rally. In 1964 it won the 2-litre GT category in the Manufacturer's World Championship and also took the prototype class win.
Results in 1965 were equally strong: victories came at several European rallies including the Spanish, Rossfeld, Hellenbronner, and Gaisburg events, plus a class win and second overall at the Monte Carlo Rally — which saw only 22 finishers from 237 starters. Class wins continued at Monza, the Targa Florio, Spa, Daytona, Le Mans, and Zandvoort, and the car repeated its 2-litre GT class championship win.
For the European Hill Climb Championship in 1965, Porsche developed the 904 Bergspyder, an open-bodied derivative based on the 904/8 coupe chassis. Stripped of its roof and fitted with a lighter open plastic body, the Bergspyder weighed approximately 120 kg less than the coupe at around 570 kg. It was powered by the same 2-litre flat-eight engine producing up to 260 horsepower at 8,800 rpm and reached a top speed of around 260 km/h. Gerhard Mitter drove the Bergspyder to victory at the Rossfeld hillclimb in the 1965 European Hill Climb Championship. Five examples were constructed, three of which were destroyed in accidents. The 904 Bergspyder was replaced at the 1965 Ollon-Villars hillclimb by the Porsche 906 Spyder Mountain.
The 904 represented a significant step in Porsche's sportscar development, introducing modern construction techniques that would become standard practice among race car builders. Its ability to compete on equal terms with far more powerful rivals established Porsche's reputation for engineering efficiency and reliability. The model was succeeded by the Porsche 906 for 1966, which adopted a tubular spaceframe with unstressed fibreglass body to address the weight inconsistency issues of the 904's spray-moulded fibreglass construction.