In the shell-model context we can interpret this as 4 $s_{1/2}$ levels, with $m = \pm 1/2$, we can also think of these are simple four pairs, $\pm k, k = 1,2,3,4$. Later on we will assign single-particle energies, depending on the radial quantum number $n$, that is, $\epsilon_k = |k| \delta$ so that they are equally spaced.