The 4AD Roadster
The 4AD is perhaps the most familiar Singer Roadster to the North American market and certainly the most plentiful.
The 4AD and 4AB are virtually identical in bodywork and general specifications and in fact they, as well as the 4AC, were all likely conceived in parallel during the 1950 model year. The 4AB was in all probability an interim model, designed to use up the remaining 9 horsepower engines in the new independent front suspension chassis. The 4AC, of course, was strictly a prototype which carried the 1,200 cc version of the SM 1500 Saloon engine . Both the 4AB and the 4AD were announced by Singer for the 1951 model year, with the 4AB destined for the home market and the 4AD for export only.
The most visible external differences between the 4AB and the 4AD were the bumpers, which were now larger and more rounded and the tail lights, which were mounted on long extension housings attached to the rear fenders. Under the bonnet, however, was considerably more power. The 4AD engine production was now fully rationalized with that of the SM 1500 Saloon, both sharing the 1,497 cc block. This provided an increase of 12 horsepower over the 4AB which, as The Autocar stated, made it " a lively open tourer with a performance that falls somewhere between that of a similarly powered saloon and an out-and-out sporting car".