The Type 4 is a bolometer or thermocouple, a device for measuring small levels of power. We also have the Type 7 and lack details for both.
The Feint etched lettering on the clear glass. The AM and crown indicate that this was the property of the UK Air Ministry. The 4 can be seen above the reference (stores code) of 10AB/1950.
The active bead is at the top and the side bars.
The actibe thermocouple. The leads that run horizontally are either just supports or a half wave dipole for picking-up the small RF field that the device was to measure.
The balloon envelope is 65 mm in diameter, and excluding the IO base pins is 138 mm tall.
Updated April 08, 2019. shape:balloon construction:pinch type:bolometer base:io pins:8 pin: