3.设等比数列{an}\{a_{n}\}{an}的各项均为正数,前nnn项和SnS_{n}Sn,若a1=1a_{1}=1a1=1,S5=5S3−4S_{5}=5S_{3}-4S5=5S3−4,则S4=(S_{4}=(S4=( )))