  We study supersymmetry enhancement from ${\cal N}=3$ to ${\cal N}=4$ proposed
by Aharony and Tachikawa by using string junctions in S-folds. The central
charges carried by junctions play a central role in our analysis. We consider
planer junctions in a specific plane. Before the S-folding they carry two
complex central charges, which we denote by $Z$ and $\bar Z$. The S-fold
projection eliminates $\bar Z$ as well as one of the four supercharges, and
when the supersymmetry is enhanced $\bar Z$ should be reproduced by some
non-perturbative mechanism. For the models of $\mathbb{Z}_3$ and $\mathbb{Z}_4$
S-folds which are expected to give $SU(3)$ and $SO(5)$ ${\cal N}=4$ theories we
compare the junction spectra with those in perturbative brane realization of
the same theories. We establish one-to-one correspondence so that $Z$
coincides. By using the correspondence we also give an expression for the
enhanced central charge $\bar Z$.
