Announced by Managing Director Spencer Wilks on 23 September 1949 the new Rover 75, then the only Rover in production, was first displayed at the opening day of the Earls Court Motor Show on 28 September 1949. It featured unusual modern styling in stark contrast with the outdated Rover 75 (P3) it replaced. Gone were the traditional radiator, separate headlamps and external running boards. In their place were a chromium grille, recessed headlamps and a streamlined body the whole width of the chassis.

The car's styling was derived from the then controversial 1947 Studebakers. The Rover executives purchased two such vehicles and fitted the body from one of them to a prototype P4 chassis to create a development mule. James Taylor's book 'Rover P4 – The Complete Story' says that this vehicle was affectionately known as the 'Roverbaker' hybrid.

Malcolm Bobbit states "The P4 set the seal on the future with a vengeance. Rover defied its critics with the P4's new look and to get some idea of the shock of the new, consider some of its rivals . . . astonishment at the P4's courageous styling." The P3 had almost no boot at all yet that had been considered rather more than adequate. The new car's bonnet-like extension to its rear was ridiculed; the driver sat well forward looking out over a relatively short bonnet and the rear wheels were set well back behind the back seat. All the new car's proportions were different from all the other new cars.

Power came from a more powerful version of the previous model's 2.1 L (2103 cc/128 in³) Rover IOE straight-6 engine now with chromium-plated cylinder bores, an aluminium cylinder head with built-in induction manifold and a pair of horizontal instead of downdraught carburetters. A four-speed manual transmission was used and the column-mounted gear lever was replaced by a floor-mounted mechanism in 1953.

A car tested by The Motor magazine in 1949 had a top speed of 83.5 mph (134.4 km/h) and could accelerate from 0–60 mph (97 km/h) in 21.6 seconds. A fuel consumption of 27.8 miles per imperial gallon was recorded. The test car cost £1106 including taxes.