∵S2＝7,S6＝91,易知q≠1
a1(1＋q)＝7
[a1(1－q^6)]/(1－q)＝91
∴[a1(1＋q)(1－q)(1＋q^2＋q^4)]/(1－q)＝91
∴q^4＋q^2－12＝0
∴q^2＝3
∴S4＝[a1(1－q^4)]/(1－q)＝a1(1＋q)(1＋q^2)＝7×(1＋3)＝28