4.已知等差数列{an}\{a_{n}\}{an},记SnS_{n}Sn为数列{an}\{a_{n}\}{an}的前nnn项和,若a1=1a_{1}=1a1=1,S7=5a5S_{7}=5a_{5}S7=5a5,则数列{an}\{a_{n}\}{an}的公差d=(d=(d=( )))