The PM4 is an audio output valve. The replacement type is the PM24M which is clearly a more modern design and gives 2.8 W in single end service. The PM4 is clearly a low power device probably with less than 500 mW audio output.
The early age of the device can be deduced from the fact that the valve carries the wording that practically no glow can be seen from the valve, indicating a dull emitter in a world used to bright emitter valves. In fact the PM4 design dates from 1925.
The above picture is of another PM4. The envelope has etched lettering and logo as well as the unlettered BBC stamp. This would place this valve to about 1927.
A second view of this valve, the moulded ridge with white paint on the base is to direct the user to the correct orientation of the valve for insertion into its socket.
The PM4 type was improved in 1929.
The balloon envelope is 42 mm in diameter and, excluding the B4 base pins, is 86 mm tall.
References: 1931 data-sheet, 1043, 4113 & 1003 Type PM4 was first introduced in 1925.