ERA R4D is the last development of this classic voiturette racing car. Originally built as R4B in 1935, the car was rebuilt as a C-Type by modifying the front end of the chassis frame to accommodate independent Porsche-type torsion bar front suspension. Over the winter of 1937-38 the car was given a completely new fully boxed frame, and was designated R4D.

As the first ERA to be fitted with a Zoller supercharger (in 1935),R4D accumulated a formidable competition record in its various guises, finally being purchased from the works by Raymond Mays, one of ERA's founders, and running as a privately entered car in 1939. Mays set numerous pre-war records in R4D, including Prescott and Shelsley Walsh hill climbs, Brighton Sprints and Brooklands Mountain circuit.

After World War II R4D continued in active competition, but the demands on Mays's time created by the evolving BRM project meant he competed less frequently. In 1952 Mays sold R4D to Ron Flockhart. In 1953 Flockhart had a phenomenally successful season, leading to his joining the BRM team as a works driver, and later successes at Le Mans and elsewhere.

In 1954 Ken Wharton purchased R4D from Flockhart and used the car to win the RAC Hill Climb Championship. In 1955 he used R4D and his Cooper to finish equal first in the hill climb championship with Tony Marsh. Since Wharton was a multiple previous winner, the RAC awarded the championship to newcomer Marsh.

From the mid-fifties onward, the car had a variety of owners, but achieved notable success in historic racing in the hands of Neil Corner and Willie Green (the latter driving for Anthony Bamford). R4D rose to pre-eminence again in the hands of Anthony Mayman, achieving many successes and setting many pre-war records at various venues. In recent years the car has been owned and driven by James Mac Hulbert, and continues to be one of the most successful pre-war racing cars still active in competition, having set new pre-war records at numerous venues.