Start by writing the electron configuration of a beryllium atom
#"Be: " 1s^2 color(red)(2)s^2#
As you can see, the fourth electron added to a beryllium atom is located on the second energy level, so
#n = color(red)(2)#
The principal quantum number,
#n#, tells you the energy level on which an electron resides inside an atom
This electron is located in the
The angular momentum quantum number,
#l#, tells you the subshell in which the electron is located.
Consequently, this electron is located in the
#m_l = 0#
The magnetic quantum nubmer,
#m_l#, tells you the specific orbital in which the electron is located.
Finally, you know that this is the second electron added to the
By convention, we take the first electron added to an orbital to have spin-up and the second electron added to the same orbital to have spin-down.
This means that you have
#m_s = -1/2#
The spin quantum number,
#m_s#, tells you the spin of the electron.
You can thus say that the fourth electron added to a beryllium atom can be described by the following quantum number set
#n=color(red)(2), l=0, m_l = 0, m_s = -1/2#
This set describes an electron located on the second energy level, in the 2s subshell, in the 2s orbital, which has spin-down.