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 dome etching is feint and the masking tape fixed to the base duplicates the anode and filament ratings. The ratings were upgraded in 1929 and with an anode voltage of 150 Volts, this is one of the later products.
The base has the anode pin indicated by an A. The anode is mounted horizontally and the blackened bulb is characteristic of the Azide process for making oxide filaments.
The dome with very faint but just legible wording.
The balloon envelope is 45 mm in diameter and, excluding the B4 base pins, is 91 mm tall.
References: 1931 datasheet, 1043 & 1003 Type PM4 was first introduced in 1925.