Number of electrons with –1/2  spin in the shell with n value 4

 

For n = 4 value of electrons is 2n ² = 32
and two electrons is filled in each orbital one have m _s value  +1/2  and other have -1/2
So for n = 4 there are 16 orbitals and 16 electrons which have m_s  value is  -1/2