Fundamental frequency of a vibrating string is expressed as
$f =\frac{1}{2L}\sqrt{\frac{T}{\mu}}=\frac{1}{LD}\sqrt{\frac{T}{\pi\rho}}$
where, D = diameter of string
$\rho$ = density of the material of string
As length L and radius are doubled, the new frequency
$f ' =\frac{1}{\left(2L\right)\left(2D\right)}\sqrt{\frac{T}{\pi\rho}}=\frac{1}{4} f$