  We have studied hadronic four-body decays of $D^+$ and $D^+_s$ mesons with a
$K_S$ in the final state using data recorded during the 1996-1997 fixed-target
run at Fermilab high energy photoproduction experiment FOCUS. We report a new
branching ratio measurement of $\Gamma(D^+\to K_S K^-\pi^+\pi^+)/\Gamma(D^+\to
K_S \pi^+\pi^+\pi^-)=0.0768\pm0.0041\pm0.0032$. We make the first observation
of three new decay modes with branching ratios $\Gamma(D^+\to K_S
K^+\pi^+\pi^-)/\Gamma(D^+\to K_S \pi^+\pi^+\pi^-)=0.0562\pm0.0039\pm0.0040$,
$\Gamma(D^+\to\K_S K^+ K^-\pi^+)/\Gamma(D^+\to K_S
\pi^+\pi^+\pi^-)=0.0077\pm0.0015\pm0.0009$, and $\Gamma(D^+_s\to K_S
K^+\pi^+\pi^-)/\Gamma(D^+_s\to K_S K^-\pi^+\pi^+)=0.586\pm0.052\pm0.043$, where
in each case the first error is statistical and the second error is systematic.
