|Length:||7.8 miles (12.6 km)|
|Meets:||A40, B4477, A4095|
|Old route now:||B4477, A4095|
|Route outline (key)|
The B4020 is a short rural B-road in west Oxfordshire.
Originally the B4020 carried straight on through the village to Black Bourton. During the 1950s the road was severed when the runways of the nearby RAF Brize Norton were extended as part of the US Air Force's facilities for the Cold War. The B4020 was rerouted to the west. Carterton itself received a bypass some years later. Then, for some reason, the road to the west of Carterton was renumbered B4477. The B4020, therefore, multiplexes west along the B4477 for a couple of miles before that road TOTSOs right and the B4020 continues on round the western end of the runways.
After passing through Alvescot the B4020 reaches Black Bourton where it rejoins its original route. It continues south across fairly featureless countryside to Clanfield, where it meets the A4095 where that road TOTSOs.