等比数列an中a1=4,前n项和Sn满足S3 S2 S4成等差数列,求an通项公式!

a1*(1-q^3)/(1-q)+a1*(1-q^4)/(1-q)=2*a1*(1-q^2)/(1-q)
解得：q=0(舍去),q=1,q=-2
q=1时,an=4,
q=-2时,an=4*(-2)^(n-1)