4.已知数列{an}\{a_{n}\}{an}的项满足an+1=nn+2ana_{n+1}=\frac{n}{n+2}a_{n}an+1=n+2nan,而a1=1a_{1}=1a1=1,通过计算a2a_{2}a2,a3a_{3}a3,猜想ana_{n}an等于((( )))