设公比为q,S3n=Sn*(1+q^n+q^2n)即1+q^n+q^2n=7,q^2n+q^n-6=0,所以q^n=2或-3,S4n=Sn*(1+q^n+q^2n+q^3n)=30或-60.
希望采纳