Hi Paul,
Welcome to the BBS. The 4 is the post code for Webley's Birmingham factory, when it was located in Weaman Street and was used between 1946 and 1958. Unfortunately there is no way of dating the pistol from the number, which is a batch rather than serial number. A replacement washer may be available from John Knibbs International/The Countrystore or you could try making a replacement yourself, which would be cheaper.
Regards,
John
Currently looking for Baikal Makarov pistols with the following prefixes to the serial number: 98, T01, T09, T21, T22
Prefer boxed or cased but will consider loose examples too.