The de Broglie wavelength of an electron in the 4th Bohr orbit of hydrogen is :

According to Bohr's and de-Broglie concept;

$$\begin{gathered} 2\mathrm{\pi r} =\mathrm{n\lambda } \\ \mathrm{\lambda }=2\mathrm{\pi r}/\mathrm{n}=2{\mathrm{\pi a}}_{\mathrm{o}}\times \frac{\mathrm{n}}{\mathrm{Z}} (\because \mathrm{r}={\mathrm{a}}_{\mathrm{o}}\times \frac{{\mathrm{n}}^2}{\mathrm{Z}} ; {\mathrm{a}}_{\mathrm{o}}=\mathrm{radius} \mathrm{of} 1^{\mathrm{st}} \mathrm{orbit}) \\ \mathrm{for} \mathrm{n}= 4 \\ {\mathrm{\lambda }}_4= 8{\mathrm{\pi a}}_{\mathrm{o}} \end{gathered}$$