Thevenin calculation
Thevenin voltage is the open-circuit voltage you "see" from the load side, after you remove the load. If you wanna calculate the thevenin voltage seen from Q2's base, then remove it.You can see that it is nothing but the voltage across the R4. The R4 and R3 form simple voltage divider here. Hence,
$$V_{th} = V_1*R4/(R_3+R_4)$$