The Southern Railway (SR) gave the designation 4-LAV to the electric multiple units built to work the semi-fast services on the route between London and Brighton.

None of these units survived long enough in British Rail ownership to be allocated a TOPS class.

The original series of 4-LAV (4-car Lavatory stock) units, (numbers 1921–1953), were built at Eastleigh carriage works during 1931 and 1932 for use on the newly electrified London to Brighton line.

The TCL (Trailer Composite with Lavatory) had five first-class and three third-class compartments seating 30 and 24, respectively, with a toilet at each end.

Almost all the units were scrapped, but some of the 1940-built cars survived in departmental (non-revenue earning) use for several more years.