Mullard describe the SP4 as an indirectly heated HF pentode for use as a speech detector, or RF amplifier, in AC mains receivers. The B5 based version was introduced in 1933 and the B7 based version a year later.
The heating time is given as 35 seconds. Equally important is its relatively thick heater (1 Amp rating) and improved heater-cathode insulation.
Apart from its ungainly size, the main weakness of the SP4 was that it was unsuitable for AGC applications and therefore had no place in the 'modern' type of superhet which became the norm from the mid-1930s onwards, using the variable-mu pentode for RF and IF amplification.
This exhibit has the later version of the shouldered or classic envelope shape with the 'top hat' top cap and the bright gold paint with the B5 base, See SP4 for the B7 base version. SP4 has the initial shouldered envelope shape and is missing the Bakelite screw type top cap that is present in SP4.
The reverse is plain.
The classic envelope is 49 mm in diameter and, excluding the B5 base pins, is 121 mm tall.
References: Datasheet & 1043. Type SP4 was first introduced in 1933.