One of the emission spectral lines for $\mathrm{Be}^{3+}$ has a wavelength of $253.4 \mathrm{~nm}$ for an electronic transition that begins in the state with $n=5 .$ What is the principal quantum number of the lower-energy state corresponding to this emission? (Hint: The Bohr model can be applied to one-electron ions. Don't forget the $Z$ factor:
$Z=$ nuclear charge $=$ atomic number. $)$