The Southern Railway (SR) gave the designations 4-COR, 4-RES, 4-BUF and 4-GRI to the different types of electric multiple unit built to work the route between London Waterloo and Portsmouth Harbour. The 4-COR type units survived long enough in British Rail ownership to be allocated TOPS Class 404. The COR designation had previously been used for the 6-PUL units and was reused by them during World War II when the Pullman car was stored, but this stock was different from the 4-COR units.

The SR electrified the London Waterloo to Portsmouth Harbour via Woking line in the mid-1930s, and full electric services commenced over the route from April 1937. For this service, 29 4-COR units (4-car Corridor units, numbered 3101–3129) and 19 4-RES units (4-car Restaurant units, numbered 3054–3072) were built.

Corridor connections were provided throughout each unit, including between units. This gave them a distinctive front-end appearance as the headcode display was placed on the opposite side of the gangway connection to the driving cab window, leading to their nickname of Nelsons.

For services from London Victoria to Portsmouth via Dorking, another 26 4-COR units (numbered 3130–3155) were built, together with 13 4-BUF units (4-car Buffet units, numbered 3073–3085). These new units followed the same design as the Phase 1 stock, except that the restaurant and kitchen facilities of the 4-RES units were replaced by the simpler provision of a buffet. 

One complete unit was saved for preservation, as were a number of individual vehicles.