When did London have only 4 digit telephone numbers?

When each exchange had an individual name, local numbers had four digits:

Whitehall 1212

Waterloo 4564

After a few years the DIRECTOR system was introduced and the names were replaced with three digit excahnge codes.

Now the number had seven digits: 456-3232

When STD dialling was introduced, London acquired the 01 area code: 01-456-3232.

With the swap to the 020 area code in 2000, the seven digit local numbers became eight digit numbers. Those in inner London were prefixed with a 7 and those in outer London with an 8: e.g. (020) 8456 3232.