The FX4 is the classic Black Cab

Hackney carriage

A hackney or hackney carriage is a carriage or automobile for hire...

. While the majority are black, there is in fact no requirement for them, or indeed any other make of London taxi to be black. Over the years, the FX4 has been sold under a number of different makers' names.
The FX4 London taxi was the successor to the Austin FX3

Austin FX3

The Austin FX3 was a taxicab designed to comply with the Metropolitan Police Conditions of Fitness for London taxicabs, but was used in other towns and cities in the United Kingdom...

, which was produced between 1948 and 1958. In its day the FX3 was the most widely used taxi in London. Like the FX3, the FX4 was designed by Austin

Austin

Austin is the capital of the U.S. state of Texas.Austin may also refer to:-In the United States:*Austin, Arkansas*Austin, Colorado*Austin, Chicago, Illinois*Austin, Indiana*Austin, Minnesota*Austin, Nevada*Austin, Oregon...

 in collaboration with Mann and Overton, the London

London

London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...

 taxi dealership that commissioned it (and paid for half of its cost) and Carbodies

Carbodies

Carbodies LImited is a British company based at Holyhead Road, Coventry. It started business as a coachbuilder, and now, as The London Taxi Company is best known for its production of London taxicabs.-History:...

, the coachbuilder that built the body and assembled the cab ready for sale.