According to Bohr's model for the hydrogen atom, the radius of the nth stationary state (orbit) is given by the expression $r_n = n^2 a_0$, where $a_0$ is the Bohr radius (radius of the first orbit, n=1) . For the second Bohr orbit, the principal quantum number $n = 2$. Substituting this value into the equation: $r_2 = (2)^2 a_0 = 4a_0$.