When the British military transitioned form the .303 British cartridge to 7.62mm NATO in the 1950s, it replaced the Enfield rifles with the new L1A1 SLR (the FAL) but retained the Bren gun as a support weapon. The Bren was updated to use 7.62mm, in a process more complicated than most people would think. Ultimately, about 16,000 thousand L4 series Brens were made, as they were rather quickly supplanted by the FN MAG as a belt-fed support weapon.

The four different patterns of L4 are:
L4A1 - the initial pattern, without magazine supports
L4A2 - the Bren MkIII in 7.62mm with magazine supports
L4A3 - the Bren MkII in 7.62mm with magazine supports
L4A4 - the A2 and A3 patterns with chrome-lined barrels