∵a8/a4=(a1q^7)/(a1q^3)=q^4=2
∴S4=[a1(1-q^4)]/(1-q)=-a1/(1-q)=4
∴a1=-4(1-q)
S8=[a1(1-q^8)]/(1-q) = {-4(1-q)[1-(q^4)^2]}/(1-q) = -4(1-2^2) = 12