I suppose the pre-insertion switch S1 is closed before the DUT Q1 will be inserted, after this will be opened. This serves that the DUT's base potential initially is at GND, so the DUT cannot get destroyed by an initially possibly too high voltage from the charged cap(s) C1 and C2, if R8's tap wasn't at GND before. After opening the pre-insertion switch, the cap(s) get slowly charged via R8 .
R4 serves for a limited discharge current - good for the cap(s) and the switch.
R2 serves for a well defined ac base emitter voltage of Q1 , resulting from the current defined by the 200mV 50Ω ac source, used as reference during the gain test. Note that the ac source's screen node (its GND reference) is connected between R2 and R3.
R3 - with open or closed parallel jumper - probably is responsible for 2 different gain adjustments, due to its negative feedback effect.