Let R₁ be the radius of the second stationary orbit and R₂ be the radius of the fourth stationary orbit of an electron in Bohr's model. The ratio $\frac{R_1}{R_2}$ is

Radius of the Bohr orbit is given by R=R₀ n², where R₀ is the radius of the 1^st  stationary orbit, n is number of stationary orbit. 
We need to find out the ratio of 2^nd to 4^th stationary orbit ratio.
$\frac{{\mathrm{R}}_1}{{\mathrm{R}}_2}=\frac{{\mathrm{R}}_0{2}^2}{{\mathrm{R}}_0{4}^2}=\frac{1}{4}=0.25$
So, ratio of radii of 2^nd to 4^th orbit is $\frac{{\mathrm{R}}_1}{{\mathrm{R}}_2}=0.25$