According to Bohr's theory, the moment of momentum of an electron revolving in $$4^{th}$$ orbit of hydrogen atom is:

According to Bohr's atomic theory, the moment of momentum (which is the angular momentum) of an electron revolving in a stationary orbit is quantized. It must be an integral multiple of $$ \frac{h}{2\pi} $$.

The formula for the moment of momentum $$ L $$ is:

$$ L = \frac{nh}{2\pi} $$

Given that the electron is revolving in the 4th orbit:

$$ n = 4 $$

$$ L = \frac{4h}{2\pi} $$

$$ L = \frac{2h}{\pi} $$

Therefore, the moment of momentum of the electron in the 4th orbit of a hydrogen atom is $$ \frac{2h}{\pi} $$.