The trick here is to realize that the #s# subshell, which is denoted by #l = 0#, contains one orbital.
As you know, the principal quantum number, #n#, tells you the energy shell in which an electron is located. The angular momentum quantum number, #l#, tells you the energy subshell in which an electron is located.
In your case, you know that you have
#n = 4 -># the fourth energy shell
and
#l = 0 -># the #s# subshell
The number of orbitals present in each energy subshell is given by the number of values that the magnetic quantum number, #m_l#, can take for a given energy subshell.
#m_l = {-l, -(l-1), ..., -1, 0, 1, ... (l-1), l}#
You can say that the #s# subshell contains a single orbital because in this case, the magnetic quantum number can take a single value.
#l = 0 implies m_l = 0#
Now, you know that each orbital can hold a maximum of #2# electrons of opposite spins, as shown by Pauli's Exclusion Principle.
Since the #4s# subshell, i.e. the #s# subshell located in the fourth energy shell, contains a single orbital, the #4s# orbital, you can say that a maximum of #2# electrons can share
#n = 4, l= 0#
