As per Kepler's law we know that
[tex]\frac{T_1^2}{T_2^2} = \frac{R_1^3}{R_2^3}[/tex]
here we know that
Time period of earth around sun is 1 year while its orbital radius around sun is 1 AU
Now here we know that another planet has time period T = 4 years and its orbital radius is R2
now from above equation we will have
[tex]\frac{1^2}{4^2} = \frac{1^3}{R^3}[/tex]
[tex]\frac{1}{16} = \frac{1}{R^3}[/tex]
[tex]R^3 = 16[/tex]
[tex]R = 2.52 AU = 3.78 \times 10^{11} m[/tex]
so above is the orbital radius