The wavelength of the energy emitted when electron come from fourth orbit to second orbit in hydrogen is 20.397 cm. The wavelength of energy for the same transition in He⁺ is

5.099 cm^-1

$E\left(=\frac{h c}{\lambda}\right) \propto \frac{Z^2}{n^2} \Rightarrow \lambda \propto \frac{1}{Z^2}$

Hence $\lambda_{He^{+}}=\frac{20.397}{4}$ = 5.099 cm