An engine with both overhead (exhaust valves) and side-valve (intake valves) configuration, with cast iron cylinders. It was a development of the RAF-1A engine, differing from it by the absence of a flywheel, which was replaced by an oil pump drive.
The Royal Aircraft Factory in Farnborough was one of the few factories at the time producing both engines and airframes. The RAF 4a engine was also manufactured by Siddeley-Deasy and Daimler Co. Ltd — both from the city of Coventry. The engine powered the RAF B.E.12, R.E.7 and RAF R.E.8 aircraft.
| Configuration | 12-cylinder V-type |
| Cooling | air |
| Compression ratio | 4.2 |
| Displacement | 13.0 l |
| Weight | 300 kg |
| Maximum power | 150 hp at 1800 rpm |
