The A4121 number was allocated in around 1926 to a short link road (which was unclassified in 1922) in the Staffordshire town of Wednesbury.
The road started on the A461 at a crossroads where the street name changed from Dudley Street to Union Street. The A4121 headed north from here along High Bullen, ending one block further north on the A462 Trouse Lane/Upper High Street.
Owing to the short length of the road it is not known how long the number lasted. In more recent years the A461 has been rerouted away from Union Street, which is now pedestrianised, and High Bullen has been heavily upgraded to become part of the D2 relief road through town. The crossroads at the southern end of the ex-A4121 is now a pedestrianised square in front of a supermarket.
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