Built of wood with an unbraced cantilever wing, the S.4 was powered by a Napier Lion engine developed to produce 700 horsepower (520 kW) over a short period.

During navigation trials on 23 October the repaired aircraft was observed to be performing well, but then, for reasons that have not been fully explained, it went out of control, and was destroyed when it dived into the sea from 100 feet (30 m), injuring the pilot, Henry Biard.

Supermarine's new design was for a mid-wing cantilever floatplane which resembled a French monoplane, the Bernard SIMB V.2, which had broken the flight airspeed record in December 1924.

[2] The S.4 was the first Schneider Trophy entrant to be supported by the British government, who agreed to buy the aircraft if Supermarine and Napier covered the initial costs of development and construction.

[7] The fuselage had a covering of diagonally laid spruce planking over plywood formers constructed around a pair of steel A-frames,[7] to which the engine bearers and wing spars were attached and which carried the floats.

[3] In September 1925, the magazine Flight reported:[11] Perhaps one may describe the Supermarine-Napier S.4 as having the appearance of having been designed in an inspired moment, but having all that is considered best in British construction incorporated in its details.

[3][15] With high hopes of a British victory in the forthcoming Schneider Trophy competition at Bay Shore Park, Baltimore, the S.4, together with two Gloster III biplanes, was shipped to the US aboard the SS Minnewaska, free of charge.

[19] Parts of the wrecked aircraft were salvaged by the doomed HMS Valerian, which had been dispatched to Baltimore from the Imperial fortress of Bermuda to support the British team.

[26] The race was won two days later by Lieutenant James Doolittle, flying a Curtiss R3C at an average speed of 232.573 mph (374.443 km/h), which was faster than the S.4's world record of a month before.

[27] It was designed with new technology, with floats that were the most advanced of their time, and a wing, with its lack of external bracing wires, that had never before been incorporated into a Supermarine aircraft.

The greatest speed increase—considered to be approximately 24 miles per hour (39 km/h)—was produced by the introduction of surface radiators to cool the engine, as they significantly reduced the drag forces acting on the aircraft.

[29] Tests made on a model of the S.4 at the National Physics Laboratory which were done after the crash revealed that the Lamblin radiators accounted for a third of the aircraft's drag and that without them the S.4 would have been the "cleanest" monoplane in the world.