K4 (designed by the Post Office Engineering Department in 1927) incorporated a post box and machines for buying postage stamps on the exterior. Only a single batch of 50 K4 kiosks were built.

Some contemporary reports said the noise of the stamp-machines in operation disturbed phone-users, and the rolls of stamps in the machines became damp and stuck together in wet weather.

Ten survive with four in public use at Frodsham, Warrington, Whitley Bay and near Tunstall, East Riding of Yorkshire.

A fine example of a K4 may also be found outside the station building at Bewdley on the Severn Valley Railway. There is a fully restored K4 (including correct telephone and coin operations) at the Bury Transport Museum (East Lancs Railway) Castlecroft Yard – Bolton Street.